In order to test the mitosis-inhibiting effect and the tissue specificity of the epidermal G2 chalone for tumour cells, extracts from hairless mouse epidermis were tested in short-term tissue cultures of cells from human respiratory tract epidermoid carcinomas and adenocarcinomas. The chalone inhibited strongly the mitotic activity in two cases of histologically proven epidermoid carcinoma, and had no effect in two cases of adenocarcinoma. In one case of a supposed epidermoid carcinoma, the chalone had no effect. Revision of the histology, and the result of autopsy 11 months later, showed that in this case the lesion in the lung had been a poorly differentiated metastasis from an adenocarcinoma of the ovary. Liver extracts produced in the same way as the epidermal extracts showed no mitotic inhibition in any of the cultures. These results indicate that epidermal G2 chalone produced from mouse skin is tissue specific for human epidermoid tumour cells, and also indicate that a chalone test might be used as a diagnostic tool for poorly differentiated carcinomas to see whether they are of epidermoid origin or not.