Vincaminorine and vincaminoreine, the monomeric indole alkaloids, belonging to the respective quebrachamine skeletal types, and vincadifformine of the aspidospermine group were those of 15 bases examined found to exhibit a considerable inhibition effect on the P388 cells. The above-mentioned secondary metabolites inhibited the synthesis of nucleic acids and proteins. Vincadifformine, which was found to inhibit most significantly the biochemical functions of P388 cells of all monomers under study, stopped the proliferation of cells in vitro in a 50 micrograms X ml-1 concentration even after 12 h of action. The effect of vincadifformine was greater than that of vinblastine, which was evaluated under the same conditions on the P388 cells in vitro.