AbstractThe effect of 14C‐labelled amino acids on three different established mammalian cell lines, i. e., on Hep‐cells, HeLa‐cells, and Chinese hamster ovary cells, was studied. The cells were incubated in the presence of 20 μCi 14C‐amino acids per 1 ml culture medium for 0–24 h or in the presence of 200 μCi/ml for 30 min. Thereafter, the chromosomes of the cells, the ability of the cells to form colonies (plating efficiency) and the protein patterns of the cells obtained by two‐dimensional electrophoresis were investigated. The results showed and increase in the number of chromosome breaks and a decrease in the plating efficiency when the incubation period was increased. In contrast, the frequency of qualitative and quantitative changes in the protein patterns of cells exposed to radioactive amino acids was not increased over the control level. Possible reasons for this discrepancy are discussed.