Cholera toxin enhanced the in vitro murine immunoglobulin M antibody response toward the T-dependent antigen, sheep erythrocytes, in a concentration-dependent manner (0.01-1.0 microgram/ml). This enhancement has been previously attributed to the cyclic AMP-elevating properties of the toxin mediated through the A subunit. However, the B subunit of the toxin, which facilitates toxin binding to cells through association with GM1 gangliosides, produced enhancement of the antibody response at concentrations that did not induce cyclic AMP generation (0.25-2.5 micrograms/ml). Time of addition studies focused the potentiation window during the first 24 hr after immunization. Neither the intact toxin nor the B subunit increased baseline proliferation of splenocytes or increased background antibody responses. Splenocytes that were preincubated with cholera toxin or B subunit and subsequently washed before immunization also elicited increased antibody responses. Preincubation of the sheep erythrocyte antigen with B subunit, however, did not increase antibody responses. Preincubation of splenocytes with the B subunit in combination with GM1 gangliosides effectively abolished the enhancement of the antibody response. Therefore, a component of the humoral immunoenhancement of cholera toxin is mediated through the interaction of the B subunit with splenocytes and is independent of the cyclic AMP-generating effects of the intact toxin.