Cellular oncogenes are expressed as an intrinsic part of the transformed or neoplastic phenotype. More than 60 of the known cellular oncogenes play a specific role in normal cellular development and differentiation. To examine the correlation between ras oncogene expression and the development of cervical cancer, this study investigated the reactivity of cervical intraepithelial neoplasia(CIN) and carcinoma of the uterine cervix by using anti-ras P21 mouse monoclonal antibody. The expression of ras oncogene significantly increased with the grade of malignancy from 11% in severe dysplasia, 30% in carcinoma in situ, 43% in microinvasive carcinoma, to 53% in invasive cancer. The expression of ras oncogene was not correlated with histologic type, tumor size, and nodal status of cervical cancer. It was concluded that expression of ras oncogene is related to early phase of carcinogenesis and tumor invasion of carcinoma of the uterine cervix.