Uterine malignant mixed mesodermal tumors (MMMT) are highly malignant tumors containing both malignant glands and stroma, while adenosarcomas (AS) are less aggressive tumors composed of malignant stroma and benign glands. Immunohistochemistry was used to grade overexpression of p53 protein, HER-2/neu protein, epidermal growth factor receptor (EGFR), and Ki-67 antigen in both the glands and stroma of tissue from 20 women with MMMT and 6 women with AS. EGFR was overexpressed in 2 AS and 9 MMMT, and was more commonly found in the sarcomatous component than the carcinomatous component in MMMT (P = 0.03). p53 was not found in any AS samples and was strongly present in 6 MMMT samples with a random distribution between the malignant components. HER-2/neu protein was not overexpressed in any AS or primary MMMT. Ki-67 antigen, a marker of cell proliferation, was found at higher levels in MMMT than AS samples (P = 0.03) and high Ki-67 antigen expression correlated with a decreased survival in patients with MMMT (P = 0.004). Independent characterization of oncogene proteins in the malignant components of these heterogeneous tumors may provide insight into the histogenesis and behavior of these malignancies.