The anti-proliferative effect of azatyrosine, a newly discovered antibiotic from Streptomyces, was examined in Balb/c-originated serum-free mouse embryo (SFME) cells and transformed ras/myc SFME cells which have activated human c-Ha-ras genes. Azatyrosine suppressed their growth in a concentration-dependent manner. Growth suppression in both cells was detectable within 2 days after culture with 250 micrograms/ml azatyrosine. Non-transformed SFME cells, however, regained rapid growth after 6 days even in the presence of azatyrosine, whereas ras/myc SFME cells did not recover from the suppression. Despite the growth inhibition of ras/myc SFME cells, expression of human ras in the cells was not inhibited by azatyrosine. Meanwhile, SFME cells have the ability to express glial fibrillary acidic protein (GFAP). This expression is induced by serum-supplemented medium, though the serum inhibits the growth of SFME cells. Azatyrosine did not induce GFAP in ras/myc SFME cells, but inhibited growth. Furthermore, azatyrosine did not induce GFAP in SFME cells, and had no effect upon the expression of GFAP induced by serum in these cells. These results suggest that azatyrosine inhibited the growth of ras/myc SFME cells through a mechanism independent of those involved in growth inhibition and induction of GFAP expression by serum in SFME cells.