Alloantigen-specific cytotoxic T lymphocyte precursor (CTL-p) frequencies were analyzed in ten patients with histologically proven breast cancer receiving prophylactic RT. The frequency of CTL-p was assessed by limiting dilution (LD) analyses before, immediately after discontinuation of treatment and at various times following RT. The number of pbmnc, adherent cells and T cells was determined in parallel. Local RT led to a minor and transient reduction of CTL-p frequencies lasting approximately three months: on average a 25% decrease of CTL-p numbers was seen immediately after RT. Three months following treatment, a 20% reduction was still evident. Values subsequently returned to pretreatment levels. Moreover, these changes in the frequency of antigen-specific CTL were accompanied by a 25% to 39% decrease in the blood T cell counts lasting for more than 12 months. The reductions following local RT were less pronounced than those induced by immunosuppressive drugs in allograft recipients.