An immunocytochemical technique has been developed for the identification in situ of immunoglobulin-producing cells in tissues fixed in Bouin's solution and embedded in paraffin. Technical details are discussed as well as the application of the technique to the study of plasma cells in the inflammatory infiltrate around cutaneous tumors. Preliminary results have been obtained with basal cell epitheliomas, squamous cell carcinomas, and malignant melanomas. IgA-producing cells were present in all tumors. IgG-producing cells were present in variable frequency, depending on the type of tumor, and IgM-producing cells were found only in basal cell epitheliomas and squamous cell carcinomas.