In view of the possible use of viruses for the immunotherapy of breast cancer, the replication of a strain of fowl plaque virus was studied in the tumor cells of 11 mammary carcinoma patients with malignant effusions. The tumor cells were obtained by centrifugation on iodamide solutions, then cultured in vitro, and infected by a fowl plaque virus previously adapted to grow in a mammary carcinoma cell line. Virus multiplication was observed in all cases, a prerequisite for the use of autologous viral oncolysates for immunotherapy in mammary carcinoma patients.