Human epidermoid carcinomas and adenocarcinomas from the respiratory tract were established as cell cultures and grown on Cell finder film slides (Microlab, Holland). The cultures showed lack of contact inhibition, and cytogenetical analyses revealed both numerical and structural karyotype anomalies, with various abnormal chromosome stemlines. Pig and mouse skin extracts supposed to contain the epidermal G2 chalone and mouse liver extracts prepared in a similar way were added to the cell cultures, which were then assayed for mitotic inhibition by means of the Colcemid technique, i.e. the number of Colcemid-arrested mitoses per 1000 cells during 4 h was counted. The results indicate that the species-non-specificity and the reversibility of the epidermal G2 chalone activity, previously demonstrated in both malignant and non-malignant animal tissues and in human epidermis, is to be found even in human epidermoid carcinomas in vitro. The reversibility of the chalone-induced mitotic depression could be demonstrated by repeated counts of the number of proliferating cells in the cultures before and after addition of chalone extrcts. Whether this hold true for malignant human tumours in general remains, however, to be seen.