The ability of colchicine (Col) to interfere with suppressor cells specific for the soluble protein antigen human gamma globulin (HGG) has been examined. This interference may be the mechanism of the adjuvanticity promoted by Col. When injected into A/J mice at the appropriate time and concentration, both Col and cyclophosphamide promoted an adjuvant increase in the plaque-forming cell response to 100 micrograms of immunogenic, aggregated HGG. Col abrogated both the induction of suppressor cells when injected with 3 h of tolerization with deaggregated (DHGG) and the expression of previously induced suppressor cells when injected with the antigenic challenge. Interference with the generation and expression of antigen-specific suppressor cells had no detectable effects on the immunologic unresponsive state to HGG. Col did not interfere with the induction of tolerance at a dose (1 mg/kg) that abolished the generation of suppressor cells. Furthermore, the absence of colchicine-sensitive-suppressor cells during the establishment of tolerance had no observable effect on the duration of unresponsivness in either helper T- or B-lymphocyte populations. Finally, Col was not able to terminate the unresponsive state established by DHGG even when responsive splenic B cells could be demonstrated in tolerant animals. These data indicate that suppressor cells are not required for the establishment and maintenance of the unresponsive state to this antigen.