We reviewed 51 serous effusions (50 peritoneal/one pleural) from 38 patients with uterine (30 cases) and ovarian (eight cases) malignant mixed Müllerian tumors (MMMT). There were 16 patients (42%) with positive effusion cytology specimens; 13 cases (81%) were diagnosed as adenocarcinoma with three cases (19%) interpreted as having a sarcomatous component. Eight of 16 positive effusion specimens had cell block material available for immunoperoxidase (IP) study that included cytokeratin (AE1/3), vimentin, muscle specific actin (HHF) and S-100 protein to determine if unsuspected mesenchymal components were present in the cases originally diagnosed as carcinoma (six cases), or sarcomas (two cases). In the six cases originally interpreted as carcinoma, three were diagnosed as adenocarcinoma and three as poorly differentiated carcinoma. All three of the cases considered adenocarcinoma and two of those diagnosed as poorly differentiated carcinoma reacted only with AE1/3 and vimentin. The remaining case, considered a poorly differentiated carcinoma, stained only with vimentin. In the two cases having cell blocks interpreted as having a sarcomatous component, only vimentin was positive in one while AE1/3, vimentin, HHF, and S-100 were positive in the other. The case where all immunohistochemical stains were reactive contained both carcinomatous and sarcomatous components. In the three cases considered sarcomatous, the cytomorphologic features helpful in the recognition of a mesenchymal component included a dissociated smear pattern of pleomorphic round to oval cells and/or spindle cells. In retrospect, the IP stains did not alter any of the original diagnoses.(ABSTRACT TRUNCATED AT 250 WORDS)