Previous successful transplantations of human tumour cells into athymic nude mice have been described when cells were injected with a reconstituted basement membrane (matrigel). We have compared the development and the histology of tumours following injection with matrigel or with culture medium of a panel of tumour cells exhibiting different degrees of tumorigenicity. Two cell lines (MCF7 and MCF7/6) required matrigel in order to form tumours. When inoculated with matrigel, all the other cell lines tested [MCF7 gpt, MCF7ras, MCF7(AZ), MCF7(AZ)TD5, MDA-MB 231, HT1080] showed increased tumour take and reduced latency period. Human primary tumours (melanoma, breast and colon cancers) were transplanted successfully into nude mice, in the presence of matrigel. Breast primary tumours or cell lines gave rise to poorly differentiated carcinomas. The other tumours presented histopathological patterns typical of differentiated human cancers.