Abstract The synthesis of RNA, DNA and protein by PHA-stimulated human lymphocytes was each inhibited to almost the same extent by ouabain at concentrations greater than 10 −8 M. The cells were equally sensitive soon or late after PHA administration. Inhibition of nucleic acid synthesis was delayed 2 h, but inhibition of protein synthesis took place more rapidly. The effect of ouabain on the cells at low concentrations appeared to be reversible. In cultures obtained from both healthy and almost all leukemic donors, 2.2 × 10 −7 M ouabain inhibited the increment of nucleic acid synthesis induced by PHA, but spontaneous RNA synthesis was only partly suppressed by ouabain. By autoradiography using 3H-uridine, it was shown that the glycoside uniformly reduced or prevented RNA synthesis in blast cells throughout the culture, and not just in certain sensitive cells. Excess potassium was highly effective in preventing the inhibitory action of ouabain; about 1.05 × 10 5 equiv. of K/mole of ouabain prevented by 50% the inhibition of DNA synthesis due to ouabain alone. The glycoside inhibited the mixed lymphocyte reaction at the same concentrations and to the same extent as it did PHA stimulation, indicating that its inhibitory effect on lymphocyte transformation was not specific to PHA-activation. Clumping and redistribution of nuclear chromatin were the major morphological changes observed in the nuclei of lymphocytes and blasts cultured with ouabain. The results are consistent with a postulated working hypothesis that lymphocyte stimulation involves an early and protracted activation of membrane transport Na-K-ATPase, and is critically dependent on a subsequent increase of intracellular potassium which is suppressed by ouabain.