As many as 100 tapes, a quarter-inch inch wide and three and a half inches in diameter, might fit into a briefcase, said Marilyn McLane, who oversees the patent microfilm collection at the Boston Public Library. Officials at one library said they suspected two men with large, brown briefcases of taking its microfilm.

At least as puzzling as questions of who committed the crime and how it was done, however, is why anyone would steal the tapes.

The Patent Library Depository Program, with member libraries in all but nine states, makes patent information available that previously was available only in Washington, said Carole A. Shores, director of the program.

''Why anyone would want to steal the tapes when they can easily access the information is beyond me,'' Ms. Shores said. ''And anyone who wants a copy of a patent can get one from us or a library for about $1.50.''

But purchasing the entire 20-year collection would cost $102,670, said Douglas N. Roesemann, vice president of marketing for Research Publications Inc., a Connecticut-based company licensed to produce and sell copies of the microfilm. Probably Business-Related

Although other possible scenarios have been suggested, ranging from foreign nationals stealing the film to fraternity brothers playing pranks, Mr. Cavanagh said the F.B.I. is operating under the assumption that the thefts were business-related.

The fact that public library collections apparently were not touched also mystifies those connected with the case, although some have theories.