Previous studies have shown that many melanoma patients have cytotoxic leucocyte‐dependent antibody (LDA) which is inhibited by factors in their sera. These factors were shown to be small molecular weight glycoproteins which could also be identified in the supernatants of melanoma cultures, and which were therefore most likely to be melanoma cell surface antigens. The present studies have shown that these blocking factors against melanoma LDA can be detected in patients with primary melanoma, and that the levels of this activity show an inverse correlation with the levels of LDA in the sera of melanoma patients. The appearance of melanoma LDA after surgery for localized melanoma was shown in approximately a third of the patients, and this pattern was shown to result from the disappearance of the LDA blocking activity after surgical removal of the melanoma. Assays were designed specifically to measure the melanoma LDA blocking factors by plasmapheresis. It is not yet known whether the melanoma LDA blocking activity has correlation with tumour growth in melanoma patients. The potential value of measuring this activity to assess response to therapy was shown by studies designed to remove the blocking factors by plasmapheresis. It is not yet known whether the melanoma LDA blocking activity has biological significance in regard to tumour growth, but these studies appear to indicate that measurement of these factors in the sera of melanoma patients may provide a usefulin vitromonitor of tumour growth in melanoma patient