Two nitroaromatics, 6-nitrobenzo[a]pyrene (6-N-BaP) and 6-nitrochrysene (6-N-CRY), and the corresponding parent hydrocarbons, benzo[a]pyrene (BaP) and chrysene (CRY), were studied in in vitro transformation assays with Syrian hamster embryo (SHE) cells, BALB/3T3 and C3H10T1/2 mouse cell lines. The three cell systems showed different sensitivities to the transforming effects of the chemicals studied, SHE cells being the most efficient, followed by 3T3 cells and the last being C3H10T1/2 cells. In the SHE cell system all compounds were active. Considering the concentrations (in microM) and the transformation frequency BaP was the most active, followed by 6-N-BaP, 6-N-CRY and CRY. In the BALB/3T3 standard assay and in the C3H10T1/2 assay only BaP was clearly active. When used as initiators 6-N-BaP and 6-N-CRY were inactive in the C3H cell system. In conclusion 6-N-BaP appears less active in in vitro systems than the parent compound BaP; 6-N-CRY is probably negative since it is questionable in vitro and negative in mouse skin.