EXHIBIT 4

JOINT FILING AGREEMENT

In accordance with Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, each of the persons named below agrees to the joint filing of a Statement on Schedule 13D (including amendments thereto) with respect to the common stock, $0.01 par value per share (including the associated preferred stock purchase rights), of MedImmune, Inc., a Delaware corporation, and further agrees that this Joint Filing Agreement be included as an exhibit to such filings provided that, as contemplated by Section 13d-1(k)(ii), no person shall be responsible for the completeness or accuracy of the information concerning the other persons making the filing, unless such person knows or has reason to believe that such information is inaccurate. This Joint Filing Agreement may be executed in any number of counterparts, all of which together shall constitute one and the same instrument.

Dated as of June 13, 2007

	ASTRAZENECA BIOPHARMACEUTICALS INC.


	By:	/s/ Chris R. W. Petty
		Name: Chris R. W. Petty
		Title: Assistant Secretary and Vice President

	ASTRAZENECA PLC


	By:	/s/ Shaun F. Grady
		Name: Shaun F. Grady
		Title: Authorised Signatory

MEDIMMUNE INC /DE SC 13D