Non-seminomatous germ cell tumours of the testis (NSGCT) form a heterogeneous group of neoplasms. Cell lines derived from NSGCT may provide useful data concerning the biology of neoplasic precursor germ cells, differentiation of tumour stem cells and the relationship between various tissue components of these tumours. Four NSGCT were studied, two mixed tumours composed of teratocarcinoma, yolk sac and trophoblastic elements, and two malignant teratomas with a massive neuroectodermal component, equivalent to primary neuroectodermal tumours (PNET) of the testis. The explanted tumours gave rise to various cell populations, including epitheloid cells, flattened large cells, spindle cells and tear drop cells of neuroblastic type. Ultrastructurally, cultured cells expressed various degrees of neural and muscular differentiation: neurosecretory granules, intermediate filaments of glial nature, and filaments resembling Z-bands. Cultured cells showed the expression of several neural and muscular markers, including neurofilaments, cytokeratin, actin, desmin, neuron-specific enolase, glial fibrillary acidic protein and HNK-1. In addition, three cases expressed HBA-71 antigen and two expressed MyoD1 protein. All cases were aneuploid, and an isochromosome 12p, i(12p), was detected in three cases. Myoblastic and neural cells are the predominant tumour cells that grow in vitro, independent of the nature and composition of the primary germ cell tumour. A histogenetic relationship between germ cell tumours and small round cell tumours of childhood is suggested.