The simultaneous administration of colchicine (CC) with a T-independent antigen, e.g. 2,4,6-trinitrophenyl-keyhold limpet hemocyanin-Sepharose, to intact animals effectively enhanced their hapten-specific plaque-forming cell (PFC) response. However, in congenitally athymic nude mice in which T-cell regulation was absent, CC was ineffective in producing enhancement. These observations suggest that the target cell acted upon by CC is most likely thymus-derived. Furthermore, the injection of CC with the co-polymer of L-glutamic acid50-L-tyrosine50 (GT) abolished GT-specific suppression of the PFC response to GT-methylated bovine serum albumin. Spleen cells from CC-treated and GT-primed hosts could no longer transfer suppressive activity to normal recipients. These results provide evidence that CC is capable of inactivating or eliminating suppressor cells or their precursors. Thus, CC-induced enhancement of the antibody response may be explained, at least in part, by its antimitotic, and hence lethal effect on dividing suppressor T cells.