The cytotoxicity and mutagenicity of several polycyclic hydrocarbons and their K-region derivatives were tested in a clone of Chinese hamster cells; the production of clones resistant to 8-azaguanine was used as the marker for mutagenesis. In the series related to benz(a)anthracene, the K-region epoxide was highly mutagenic, and the phenol was less mutagenic; the hydrocarbon and cis and trans-dihydrodiols were not mutagenic. Seven resistant clones were isolated and retained their drug-resistance; three of these could be reverted to the wild type. There was no difference in the chromosome numbers among the parent and mutant clones. The results in the methylcholanthrene series were similar to those for benz(a)anthracene. However, in the dibenz(a,h)anthracene series, the phenol was more mutagenic than the epoxide. 7-Methylbenz(a)anthracene epoxide and 7-bromomethylbenz(a)anthracene were highly mutagenic, 7-bromomethyl-12-methylbenz(a)anthracene was less mutagenic, and the parent hydrocarbons were inactive. These results demonstrate that metabolic activation of polycyclic hydrocarbon is required for mutagenic activity in mammalian cells.