The FBI is trying to determine why two Canadians tried to obtain dangerous tetanus and botulism cultures from a Rockville firm, an FBI spokesman said yesterday.

Kevin T. Birch, 36, of Toronto, a marketing supervisor for a Canadian telephone company, and James B. Cahoon, 35, a financial consultant, of Mississauga, were arraigned in Buffalo last week on conspiracy and fraud charges in connection with the attempted purchase from the American Type Culture Collection.

"We're trying frantically to determine what this was all about," said Special Agent John H. Thurston of the FBI's Buffalo office. " . . . The biggest hole at this point[is] the motive. This is not your usual fraud-by-[telephone] wire case."

Thurston said telephone orders for the cultures were placed with the nonprofit research facility, which provides disease strains to scientists and researchers.

A Sept. 25 order for one ampule of frozen tetanus culture was made by a man identifying himself as an officer of a Canadian research company. The culture was shipped to a Buffalo suburb and is missing, said Thurston. ATCC officials later contacted the FBI after they learned that the buyer had used a bogus name.

An order for unfrozen tetanus and botulism cultures was placed about a month later. ATCC filled the order with a harmless material.