Dengue virus-infected mice showed a depressed antibody response to polyvinylpyrrolidone (PVP) when compared to controls. In both control and dengue virus-infected animals which were treated with anti-thymocyte serum (ATS) and primed with PVP, there was a heightened antibody response to PVP, suggesting that the anti-PVP response was controlled by T suppressor cells. The increase in the anti-PVP response in dengue virus-infected, ATS-treated animals was found to be similar to that seen in ATS-treated controls. T cells from infected animals could transfer suppression of anti-PVP response to normal mice, whereas the T cells from control animals could not induce significant suppression. The T cells from dengue virus-infected animals which had received 2,4-dinitrofluorobenzene (DNFB) tolerogen could induce in normal mice a significantly higher percentage of tolerance to contact sensitivity to DNFB when compared to the control T cells. The adherent and B cells from both infected and control animals failed to induce significant tolerance. These findings suggested that during dengue virus infection, there is enhanced T suppressor cell activity regulating the B cell response to PVP and T cell response to DNFB.