The generation of mouse cytolytic T lymphocytes (CTL) in primary or secondary mixed leukocyte cultures (MLC) is greatly enhanced by the addition of 2-mercaptoethanol (2-ME) to the culture medium. This enhancement can be equally demonstrated with either reduced or oxidized 2-ME. Addition of 2-ME to MLC as late as 3 days after the initiation of the culture results in a peak CTL response which is nearly as high as the one in cultures containing 2-ME throughout the 4 day-incubation period. Quantitative analysis of the CTL response in MLC supplemented with 2-ME on day 3 shows a 50-fold increase in lytic activity within 24 h, suggesting that cell differentiation, in addition to proliferation may be affected by this agent.