The mucinous-like carcinoma-associated antigen (MCA), a new tumour marker, was dosed in 150 serous effusions. The levels of MCA were measured by means of an enzyme immunoassay (EIA). Ninety-seven were pleural effusions, 49 were peritoneal effusions and 4 were pericardial effusions. Thirty-five effusions were caused by breast carcinoma, 19 by ovarian carcinoma, 17 by pulmonary carcinoma, 19 by various cancers, while 60 effusions turned out to be of non-neoplastic origin. The results were considered in comparison with the cytologic examination, and the data obtained were also elaborated in terms of sensitivity, specificity, predictive value and test efficiency. The EIA showed satisfactory statistical parameters principally regarding ovarian metastatic cancer. On the other hand, less satisfactory results were extrapolated on breast and lung cancers. Even if further studies are necessary, for example to compare MCA with other seric tumour markers, this experience suggests that MCA could be a satisfactory tumour marker in the diagnosis of metastatic ovarian effusions.