The IgA concentration and total lymphocyte count were measured in 42 women with axillary node metastases at mastectomy. Recurrence after irradiation was significantly correlated (P =.001) with an IgA level of less than 200 mg/100 ml and lymphocyte count of less than 1,500/cu mm. Low IgA concentrations present preoperatively are unaffected by therapy. Patients not irradiated showed no correlation between IgA concentration and recurrence, but irradiated women with a high level of IgA apparently remain tumor-free. We conclude that post-mastectomy irradiation is detrimental to patients with a low IgA concentration and may improve survival in women with a high pre-operative IgA level.