Twelve compounds representative of diverse classes of chemicals were evaluated for their cytotoxicity and transforming ability of human skin fibroblasts in vitro in the presence and absence of human liver S-9 mix. In the absence of the human liver S-9 mix, only seven of the twelve compounds were highly cytotoxic in the 0-100 microgram/ml range and their order of cytotoxicity was: 2,5-bis(1-aziridinyl)-3,6-bis(carboethoxyamino)-1,4-benzoquinone (AZQ) > cis-platin > N, N-bis(2-chloroethyl)-2-naphthylamine > bis(chloromethyl)-ether (BCME) > acrylonitrile > styrene oxide > aflatoxin B1 (AFB1). The other five compounds, aflatoxin B2 (AFB2), methylmethacrylate, 1-naphthalamine, 2-naphthylamine, and cyclophosphamide, exhibited less than 40% inhibition of colony formation even at 100 microgram/ml of the compound (the maximum concentration of AFB2 used was 50 microgram/ml due to its poor solubility). Cytotoxicity was not significantly altered in the presence of human liver S-9 mix except for AFB1 and styrene oxide treatment. There was a drastic increase in cytotoxicity following treatment with 1 microgram/ml or more AFB1 and a significant reduction in cytotoxicity following treatment with 100 microgram/ml styrene oxide in the presence of S-9 mix. Anchorage independent growth (AI) of treated cells in soft agar was used as a biological end point for the expression of chemical transformation. (Copyright (c) 1990 Princeton Scientific Publishing Co., Inc.)