For the Tufts University medical school, one dose of Sen. Charles E. Grassley is apparently enough.

Tufts has scheduled a conference for May 13 on the subject of conflicts of interest in medicine and research, and it invited Mr. Grassley, a Republican of Iowa who has led a series of high-profile investigations into the issue, to participate. Mr. Grassley then passed the invitation to Paul D. Thacker, a staff member who has led the senator’s investigative efforts.

But when senior Tufts officials learned about the plan, they refused to let other university administrators participate on the conference panel alongside Mr. Thacker, The Boston Globe reported today. The invitation to Mr. Thacker was then withdrawn by conference organizers.

A spokeswoman for the Tufts medical school, Christine Fennelly, said the university decided it didn’t want its administrators to publicly debate Mr. Thacker at the same time he is helping to lead an investigation into Tufts faculty members. Mr. Grassley wrote to the president of Tufts, Lawrence S. Bacow, on February 17 seeking financial disclosures for Helen W. Boucher, an infectious-disease specialist at Tufts.

For now, the May 13 conference is going ahead, without both Mr. Thacker and a conference organizer, Sheldon Krimsky, an environmental-policy professor at Tufts who is co-chairman of the university’s Committee on Ethics. Mr. Krimsky, in an e-mail message obtained by the Globe, wrote that he had recused himself after feeling his role as organizer had been “compromised,” and the university’s commitment to academic freedom diminished, by the refusal of Tufts administrators to accept Mr. Thacker’s presence. —Paul Basken