Fifty-eight metastases of malignant melanoma in 27 consecutive patients were evaluated by immunohistochemistry using monoclonal antibodies (MAb) 96.5, 48.7 and 4.2, all with a high specificity for melanoma tumour associated antigens. MAb OKT3 directed to human T-lymphocytes was used as control and normal mouse serum as background. Different degrees and patterns of staining were recorded using a scale of 0 to 4. 85% of the metastases were positive (score greater than +2) to 96.5 or 48.7 and 78% to antibody 4.2. All metastases were positive for at least one of the three antibodies, 93% for two, and 59% for all three antibodies. No substantial heterogeneity (more than +1 difference) was found in two sections approximately 5 mm apart within the same metastasis (one out of 39 metastases). In 15 patients in whom more than one tumour was examined, heterogeneity between individual metastases was found in one patient for antibody 96.5, three for 48.7 and none for 4.2. The existence of immunological heterogeneity in melanoma metastases must be taken into account when designing techniques for radioimmunoimaging and radioimmunotherapy.