Rat monoclonal antibodies were prepared against antigens expressed by uveal melanomas. Uncultured cells from primary human uveal melanomas were used for the rat inoculations and for the screening of hybridomas by enzyme-linked immunosorbent assay (ELISA). One of the monoclonal antibodies, designated 4A3, recognised a cytoplasmic antigen which was relatively specific for melanoma cells and which could be detected by immunohistochemistry in formalin-fixed, paraffin embedded tumour tissue. Western blotting showed the antigen to have a molecular weight of approximately 55-60 kD, with a doublet configuration which showed inter-tumour variation. The antigen was also detected by Western Blotting in the subretinal fluid of patients with uveal melanoma.