The effect of methotrexate (MTX) on transformation and mitosis of normal lymphocyte cultures was investigated. Mitosis was 1000-fold more sensitive to MTX than was transformation; mitosis could be prevented without any observable effect on transformation. The antimitotic effect of MTX depended on its concentration, duration of contact with lymphocytes, as well as on the developmental stage of the lymphocytes at the time of exposure. Incorporation of labeled precursors into protein, DNA, and RNA proceeded at normal rates unaffected by the presence of MTX. Transformation was partially inhibited by high concentrations of MTX (above 75 µg/ml). Folinic acid abolished the antimitotic effect of MTX, with the number of cells undergoing division directly proportional to the quantity of folinic acid added.