The immunosuppressive activity of several lactonic, nonlactonic, and heterocycle-fused cyclolignans has been demonstrated for the first time by use of a T-cell-mediated immune response. Of the compounds tested, 4'-demethyldeoxypodophyllotoxin (8), beta-apopicropodophyllin (6), and the isoxazoline-fused cyclolignan 15 are the most potent with respect to their suppression of activated splenocytes.