Primary peritoneal malignant mixed müllerian tumors (MMMTs) are extremely rare and highly aggressive malignancies associated with poor clinical prognoses. We present a clinicopathologic review of three cases of this rare tumor by examining expression of selected oncoproteins by immunohistochemistry. Three consecutive cases of primary peritoneal MMMT were examined by paraffin immunohistochemistry for expression of p53, p16, BCL2, CerbB2, and classical cadherins E-cadherin, P-cadherin, and N-cadherin. All three cases expressed p16, but showed less consistent expression of other markers, with one case expressing p53 and one expressing BCL2. All cases were negative for membrane expression of Cerb-B2. The three classical cadherins were expressed in two cases with one case showing only weak N- and P-cadherin expression. No difference in antigen expression was seen in the epithelial compared to sarcomatous components. We conclude that p16 may be a common tumor suppressor gene expressed in peritoneal MMMT. P53 overexpression may be of lesser frequency in peritoneal MMMT compared to MMMT from the ovary and the uterus. We did not observe any difference in antigen expression between areas of epithelial or sarcomatous differentiation, which would support a single pluripotential malignant clone in the histogenesis of these tumors.