The subsets of lymphocytes infiltrated into mouse tumor tissue exposed to irradiation were analyzed by the avidin-biotin-peroxidase procedure using monoclonal antibodies. The effect of PSK administration was also analyzed. Microscopic observation revealed the intense lymphocytic reactions in the exposed tumor, especially 7 days after irradiation of 20 Gy, which mainly consisted of anti-L3T4 positive cells, whereas the degree of infiltration was lower in the nonexposed tumor. PSK was found to enhance T lymphocyte subset infiltration into the exposed tumor tissue.therapy has not been analyzed clearly. Mention is also made of these mechanisms.