Requirements for arginine are different from leucine for the growth of HeLa cells in monolayer and suspension culture. Cells grow increasingly more slowly as arginine levels fall below millimolar. Most cells died at 10(-5) M in static cultures, but could be sustained in perfused cultures, but at 10(-6) M neither perfusion nor increased volume in static cultures compensated. Cell died within 3-4 days in 10(-6) M in the same manner as those in complete arginine deprivation, i.e. considerably faster than with leucine deprivation. Arginine restriction produced by arginase or arginine decarboxylase addition to culture medium gave similar results. Citrulline substituted for arginine, but ornithine and polyamines did not. Arginine was depleted 3-4 times faster from the medium than other amino acids, <5% being consumed in protein synthesis, and arginine released by protein turnover was less efficiently reutilised than leucine. Deprivation reduced protein and DNA syntheses, greatly extended S-phase and protracted the cell cycle in HeLa cells for more than leucine deprivation. The inability of the cells avoid reinitiation of S-phase resulted in their proliferative impetus driving them into an late cycle (premitotic) death. In contrast, normal human diploid fibroblasts reached quiescence with little delay and survived for >11 days. Arginine deprivation is discussed as a selectively means of tumour cell destruction.