Airborne contaminants have been implicated as important contributing factors in the aetiology of tumours of the respiratory tract. A major concern is that the potential carcinogenic activity of the particulate matter may be enhanced by the volatile material, which is adsorbed onto the surface. The cytotoxic potential of complex particulates was evaluated using the Neutral Red uptake method in primary cultures of hamster tracheal epithelial cells. Cytotoxicity of the reference particulates (benzo[ a]pyrene adsorbed on to the surface of alumina A, alumina B, ferric oxide and titanium dioxide) was increased when cultures were treated with adsorbed rather than desorbed (plus dimethyl sulfoxide) samples. Foundry fume samples did not show the same effect.