Three new established human germ cell tumour lines, H 12.1, H 12.5 and H 12.7, are described. Cytogenetic and growth characteristics, morphology, histology, and tumour marker and steroid hormone production in vitro and/or in vivo revealed properties commonly found in other germ cell tumour lines and in patients with these tumours. In vitro oestrone and oestradiol were produced by the H 12 lines and four other lines (833 KE, 1156 Q, 1428 A and 2102 EP) under high and low density (differentiation inducing) conditions. AFP was produced by one line and beta-hCG by six of seven lines under low density conditions. The pattern of oestrogen production suggests that these hormones could be useful in AFP and beta-hCG negative patients. Differentiated elements of somatic and extraembryonic character, observed in tumours of H 12.1 and H 12.7, underline the value of these lines in the study of differentiation and other germ cell tumour related questions.