Summary.-Selected monoclonal antibodies (McAbs) isolated after immunization of rats with a human colonic carcinoma membrane preparation, have been screened on frozen and paraffin sections of colonic tissue, using immunohistochemical techniques, in order to provide additional information with regard to specificity and crossreactivity with normal tissues.Of 10 McAbs previously shown to bind to a colonic carcinoma membrane preparation in a radioimmunoassay, 7 show specific staining when tested by indirect immunofluorescence on crysotat sections of colonic tissue. Three of these 7 show activity on both normal and malignant colonic epithelium, and the remaining 4 stain normal epithelium, with little or no activity on malignant tissue. In the indirect immunofluorescent and immunoperoxidase techniques on paraffin sections of the same material, only 2 McAbs retain activity; one detects an antigen in colonic mucus, and the other recognises an antigen which is sparse on normal colonic epithelium and abundant on colonic tumours.We conclude that screening of McAbs on frozen tissue sections, using indirect immunofluorescence, is a useful adjunct to conventional screening methods, e.g. binding to membrane preparations and/or cell lines in a radioimmunoassay. These techniques distinguish McAbs with similar binding values in conventional assays, identify their activity on a wide range of normal and malignant tissues, demonstrate antigens that are lost or gained in malignant transformation and finally assist in the selection of McAbs for further extensive study before possible clinical use.