The amino-thiol-containing compound D-penicillamine is used as a disease-modifying drug in the treatment of rheumatoid arthritis. In the present study the effects of D-penicillamine on Chinese hamster ovary cells (CHO) were evaluated. It could be shown that only high doses, i.e. doses exceeding the therapeutically occurring plasma concentration many times, were toxic to the cells, leading to an early death of the cells. High doses (1,500 micrograms/ml) of D-penicillamine caused an inhibition of the endogenous pyrimidine synthesis of the CHO and lead to a drastic reduction of the cellular protein biosynthesis. The observations made are discussed with respect to formerly published data about mutagenicity and carcinogenicity of D-penicillamine.