Two major xenobiotic metabolizing sub-families of cytochrome P450 (cytochrome P450 1A and cytochrome P450 3A) have been identified in soft tissue sarcomas. Cytochrome P450 1A was present in 70 per cent and cytochrome P450 3A was present in 78 per cent of tumours, respectively. A high proportion (86 per cent) of those tumours which contained cytochrome P450 1A or cytochrome P450 3A demonstrated co-expression of both sub-families. In each tumour, cytochrome P450 immunoreactivity was identified in all tumour cells and there was no intra-tumour heterogeneity. These results indicate that expression of cytochrome P450 is a common molecular event in soft tissue sarcomas and that the presence of different sub-families of cytochrome P450 has implications both for the pathogenesis and for the treatment of these tumours. Cytochrome P450 expression may influence the intrinsic drug resistance of these tumours and also provide a molecular target for anti-cancer drugs which can be activated by cytochrome P450.