The number of leukocytes, proportion and absolute number of granulocytes, lymphocytes, CD4+ cells, CD8+ cells, CD16+ cells, B-lymphocytes, monocytes, natural killer cell (NK) activity, and granulocyte and monocyte phagocytic functions--ingestion and intracellular killing--were determined in a group of 27 patients with ductal invasive breast carcinoma, stage I-III, before and 7 months following postsurgical telecobalt radiotherapy, divided into two subgroups, one of them receiving tamoxifen (TMX group) and the other one not receiving any further therapy (control group). In control group, proportion of all lymphocytes and CD8+ cells as well as absolute number of all lymphocytes, CD4+, CD8+, CD16+ and B lymphocytes were decreased following TCT in comparison to their pre-TCT values, while in TMX group only absolute number of all lymphocytes remained decreased following TCT. Moreover, post-TCT proportions of all and CD8+ lymphocytes as well as absolute numbers of all and CD4+ and CD8+ lymphocytes in TMX patients were significantly increased in comparison to the same parameters in control post-TCT patients, although there was no difference between the two subgroups before TCT. At the other hand, granulocyte ingestion was decreased in post-TCT TMX patients compared to post-TCT values in control patients and NK cell activity showed a similar, although statistically not significant, tendency. It seems that TMX helps recovery of lymphocyte populations decreased by radiotherapy, probably by stimulation of cells carrying estrogen receptors, but its effects on phagocytic functions and probably NK cell activity seemed to be rather inhibitory than stimulatory.