Abstract Indirect immunofluorescence and flow cytometry were used to determine reactivity of workshop antibodies with a large panel of human cultured cell lines which included small cell lung carcinoma (SCLC) “classic”, and “variant”; and non-small cell lung carcinomas (NSCLC). Other cell lines were SHP-77 1 (an unusual “oat cell” large cell variant which has the morphology of a “variant”, but biochemical properties of a “classic” SCLC). Also studied were 2 neuroblastoma lines, 2 colon carcinoma and 5 lymphoblastoid lines. Three morphologically similar “classic” SCLC were phenotypically different in reactivity with workshop antibodies, suggesting that testing with such a panel is a new means to classify SCLC. The “classic” lines also differed in intensity of labelling with some of the reactive monoclonals, indicating differences in density of surface markers. Xenografts of human tumour lines grown in nude mice reacted very strongly with (FAB 1) 2 anti-mouse fragments, suggesting that such xenografts have nude mouse immunoglobulins on their surface. Evidence is presented that some monoclonals may react with tumour cells via Fc receptors NCSCLC, colon carcinoma, and lymphoblastoid lines also react with some workshop antibodies.