Methyl methane sulfonate (MMS) given ip five hours before Friend Leukemia Virus (FLV) injection enhanced the leukemogenic action of FLV in virus-sensitive SJL/J mice and also in relatively virus-resistant B10SJF1 mice. MMS also decreased the humoral immune response, as measured by plaque forming cell assay. However, the timing of the effect of MMS on the immune system did not coincide with the timing of the MMS related potentiation of leukemia. Hence, it is suggested that the potentiating effect of this chemical on viral leukemogenesis is more likely due to events occurring at the intracellular level than at the level of humoral immune response. © 1979, SAGE Publications. All rights reserved.