EXHIBIT 5.1

[Gross Hartman LLC Letterhead]

May 2, 2011

Array BioPharma Inc.

3200 Walnut Street

Boulder, Colorado 80301

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection with the sale and issuance by Array BioPharma Inc., a Delaware corporation (the “Company”), of up to 10,135 shares of the Company’s Series B Convertible Preferred Stock, par value $0.001 (the “Preferred Shares”), pursuant to the Registration Statement on Form S-3 (Registration Statement No. 333-155221) (the “Registration Statement”), filed with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Act”), the related prospectus dated December 4, 2008 included in the Registration Statement (the “Base Prospectus”), and the prospectus supplement relating to the Preferred Shares to be filed with the Commission pursuant to Rule 424(b) promulgated under the Act (the “Prospectus Supplement” and, together with the Base Prospectus, the “Prospectus”). All of the Preferred Shares are to be issued and sold by the Company as described in the Registration Statement and Prospectus.

In connection with this opinion, we have examined and relied upon the Registration Statement, the Prospectus, the Company’s Amended and Restated Certificate of Incorporation, as amended, and Amended and Restated Bylaws, as currently in effect, and the originals or copies certified to our satisfaction of such other documents, records, certificates, memoranda and other instruments as we deem necessary or appropriate to enable us to render the opinion expressed below. We have assumed the genuineness and authenticity of all documents submitted to us as originals, the conformity to originals of all documents submitted to us as copies thereof and the due execution and delivery of all documents where due execution and delivery are a prerequisite to the effectiveness thereof. We have assumed that the Certificate of Designation for the Company’s Series B Convertible Preferred Stock, par value $0.001, shall have been filed with the Secretary of State of the State of Delaware.

Our opinion is limited to the matters stated herein and no opinion is implied or may be inferred beyond the matters expressly stated. Our opinion herein is expressed solely with respect to the federal laws of the United States and the General Corporation Law of the State of Delaware. Our opinion is based on these laws as in effect on the date hereof, and we disclaim any obligation to advise you of facts, circumstances, events or developments which hereafter may be brought to our attention and which may alter, affect or modify the opinion expressed herein. We are not rendering any opinion as to compliance with any

federal or state antifraud law, rule or regulation relating to securities, or to the sale or issuance thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that: (i) the Preferred Shares, when sold and issued in accordance with the Registration Statement and the Prospectus, will be validly issued, fully paid and nonassessable, and (ii) the common stock issuable upon the conversion of the Preferred Shares, when issued upon the conversion of the Preferred Shares in accordance with the terms thereof, will be validly issued, fully paid and nonassessable.

We consent to the reference to our firm under the caption “Legal Matters” in the Prospectus Supplement and to the filing of this opinion as an exhibit to a Current Report of the Company on Form 8-K. In giving this consent, we do not thereby admit that we are an “expert” within the meaning of the Securities Act of 1933, as amended.

Very truly yours,

Gross Hartman LLC

/s/ Gross Hartman LLC

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