The response of Saccharomyces cerevisiae cells to the toxin produced by certain strains of Kluyveromyces lactis was studied. The toxin caused an arrest of sensitive cells in the unbudded (G1) phase of the cell cycle, consistent with the accumulation of cells with an unreplicated (G1) content of DNA in treated populations. However, toxin-treated cells were not proficient for mating. The effects of the toxin were dependent on its continuous presence for over an hour and removal of cells into fresh medium at earlier times prevented inhibition. Following toxin treatment, cells increased in volume and continued to synthesize protein and RNA, suggesting that they were able to continue growth in the absence of division. However, several lines of evidence suggested that the toxin does not simply block proliferation in G1, but that another continuous or post-G1 event is also affected. Possible models to explain these observations are discussed.