A prospectively randomized clinical study was performed with breast carcinoma patients to determine the correlation of defined parameters of the cellular immunity and beta-endorphin plasma levels after mistletoe lectin (ML-1) standardized therapy. The subcutaneous administration of optimal ML-1 dosages (0.5-1.0 ng ML-1/kg body weight; twice a week) induced an increased beta-endorphin plasma level, enhanced activity of peripheral blood natural killer (NK-)cells and T-lymphocytes (expression of CD-25/interleukin-2 receptors and HLA/DR-antigens). Statistical analysis of the data (Spearman correlation) revealed a significant correlation between NK- and T-cell activity and beta-endorphin plasma level. Thus, an obvious correlation between immune system and the neuroendocrine system may be anticipated which might gain therapeutical relevance.