The reactivity spectrum of an anti-CALLA/CD10 monoclonal antibody for cutaneous melanoma was analysed by immunohistochemistry in a series of lesions of different Breslow thickness. Similar proportions of small primary tumours, advanced primary tumours and metastatic lesions were found to express CALLA/CD10 (31-47%). However the proportion of stained cells within a given lesion increased with tumour progression. Up to 23% of the advanced primary lesions (> 3.0 mm) showed 26-50% cells stained with the anti-CALLA/CD10 antibody and up to 14% of the metastatic lesions showed 76-100% stained cells. The expression of CALLA/CD10 was further analysed in 15 ocular melanoma lesions of different histiotype. All five spindle type lesions, three of six epitheloid and two of five mixed type lesions stained positively with the anti-CALLA/CD10 antibody. The percentage of stained cells within a given lesion varied from 30% to 100%. A total of 63% of the ocular melanomas and 38% of the cutaneous melanomas tested expressed CALLA/CD10. Experiments with cultured melanoma cell lines showed that the surface expression of CALLA/CD10 can be modulated in vitro by treatment with interleukin 2 (IL-2) and an adenosine 3',5'-cyclic monophosphate (analogue).