The effect of sodium fluoride on the growth of two continuous human cell lines, i.e. HeLa cells and human conjunctiva clone 1-5C-4 cells, was studied. The growth of HeLa cells and clone 1-5C-4 cells was arrested nearly completely by the addition of 0.95 and 1,90 mM of sodium fluoride, respectively. DNA synthesis in HeLa cells, determined by incorporation of 3H-thymidine, was not affected appreciably for the first 24-hr period after the addition of sodium fluoride. Markedly reduced incorporation, however, occurred during the next 24-hr period. Thus, there was a discrepancy between the immediate cessation of cell division and the delayed suppression of DNA synthesis. On the other hand, a suppressive effect of sodium fluoride on protein synthesis determined by 14C-leucine incorporation was evident already during the first 24-hr period. The results indicate that the inhibition of protein synthesis is the main cause of growth inhibition.