Arginase activity was measured in ten different cell lines and all of them showed arginase activity. However, the amount of the activity in the cells varied more than 1000-fold. The cell density did not appear to affect the arginase activity much, since cultures of human fibroblasts grown to different cell densities exhibited only small variations in arginase activity. Arginase activity was in general higher than that of ornithine decarboxylase and there was no correlation between the two activities. When human fibroblasts were stimulated to proliferate in serum-free medium by adding certain growth factors the activity of ornithine decarboxylase increased markedly prior to the DNA synthesis, while the arginase activity increased more slowly and to a much smaller degree. Two cell lines, baby hamster kidney and mouse 3T3 cells, had low arginase activity when adapted to grow in serum-free medium, but in spite of this they were able to grow in serum-free medium without exogenous ornithine.