Cytologic examination of exfoliated cells in body fluids is an extremely valuable method for the early detection of cancer. Nonetheless, there remains a large area of uncertainty in the diagnosis of cytologic preparations, especially of sputum, bronchial washings and urine, where cellular morphologic changes of early malignancy cannot be distinguished from changes due to inflammation and regeneration. The objectives of this study are to provide an immunocytologic technique to detect carcinoembryonic antigen (CEA) on exfoliated cells as an adjunct to the cytologic distinction between malignant and benign atypical changes. Using an antiserum to human CEA, the immunoperoxidase staining method yields a dark precipitate at sites containing CEA while preserving light microscopic cytologic detail in conventionally fixed smears, cytocentrifuge preparations and cell blocks. Prior studies of tissue sections and a pilot study of cytology cell blocks have shown a very high correlation between cell-bound CEA and proven malignancy, including carcinoma-in-situ, among lung, intestinal, ovarian and bladder neoplasms. The results of the CEA-immunocytology will be compared to subsequent biopsy and clinical follow-up data in a 3-year study. It is expected that many cytologies now diagnosed as "suspicious" or "atypical" will be more accurately evaluated, resulting in earlier detection of curable cancers.