Prompted by the observation of retroviral particle formation in teratocarcinoma cell lines and the consistent finding of antibodies against Gag and Env proteins encoded by human endogenous retrovirus (HERV)-K genomes in the sera of patients with classical seminoma, we studied ovarian and testicular germ cell tumours, their precursor lesions, dysgenetic gonads, and trophoblast lesions for expression of HERV-K sequences by in situ hybridization using radioactive and non-radioactive probes.