Dear Mr. Jones:
I was flattered and honored to receive an offer of employment for the summer of 200* at your firm. I enjoyed this past summer at the firm, and have learned much from the professional and support staffs. Unfortunately, I must decline your offer. I have always had the desire to become involved in public interest law, and I hope to spend next summer working in this field in order to learn more about it, and to determine if this is the ultimate direction that my career path should take. I feel that only by working in both the private and public law sectors will I be able to make an informed choice regarding which field I should enter upon graduation from law school.
Thank you again for the opportunity you offered. My decision to decline the opportunity to work with your fine people was difficult to make. However, I wanted to let you know of my choice as early as possible so that you could better plan for the fall interviewing season.
I will look for you when you are on campus this fall.
Sincerely yours,
Your name