Three tumours which arose in two (one male and one hermaphrodite) out of 63 chimaeric mice resulting from injection of E14TG2a embryo stem (ES) cells into host blastocysts have been investigated. All of the tumours appeared within the first 3 weeks after birth. The tumour in the male chimaera and one of the tumours in the hermaphrodite were in the perigenital region but were extragonadal. The third, smaller tumour in the hermaphrodite was on the caecum. The perigenital tumour in the male chimaera was a teratocarcinoma with a wide variety of differentiated tissues, including non-pigmented retina, as well as nests of undifferentiated embryonal carcinoma (EC) cells with high levels of alkaline phosphatase activity. The perigenital tumour in the hermaphrodite was a teratoma, less differentiated and with no evidence of EC cells. Glucose phosphate isomerase isozyme analysis indicated that both perigenital tumours were predominantly of the injected ES cell rather than the host blastocyst type. The possible origins of these tumours, which are the first reported to have arisen from ES cells in chimaeric mice, are discussed.