Data on cellular immunity of 39 patients with breast cancer and 29 patients with malignant melanoma, is presented. T/B lymphocyte ratios, lymphocyte blastogenesis (LB) with PHA and with cancer antigen preparations were carried out from two to four weeks after cancer surgery, and immediately after eight weeks of immunostimulation with a tumor-specific vaccine. Distinct differences were observed. Small breast cancers do not seem to evoke a host immune response, while small melanomas are associated with a state of immune stimulation. After immunotherapy LB in these breast cancer patients shows significant stimulation while LB in the melanoma patients changed from stimulation to normal response. Quite different results were obtained in patients with Stage II cancers; breast cancer patients showed immune stimulation which did not change after immunotherapy. The melanoma patients showed low normal immune response, which changed to high normal after immunotherapy. Patients with Stage III breast cancer and melanoma were similar in their poor immune responsiveness. Possible mechanisms are offered for these differences.