Lymphocytes from cancer patients and normal individuals demonstrated blastogenesis with allogeneic potassium chloride (3 M KCI) extracts of breast carcinoma cells. Normal individuals reacted with a greater frequency and stronger blastogenic responses to tumor extracts than did breast carcinoma patients; allogeneic extracts may have elicited recognition of normal alloantigens rather than tumor-associated antigens. Normal individuals also responded to 3 M KCI extracts of allogeneic pooled normal leukocytes, normal breast tissue, and other cancers, but did not react to extracts of autologous leukocytes.