[en] Female C3H/He mice aged 12 weeks, transplanted MM46 tumor, were used in this study on the timing of administration of immunomodulators, such as PSK (a proteinbound polysaccharide prepared from Coriolus versicolor), OK-432 (Streptococcal preparation), bestatin (inhibitor of aminopeptidases of microbial origin) and levamisole (a chemical substance belonging to anthelmintics) combined with two fractionated local irradiation with the total dose of 3,000 rads. The daily dose of 250 ng/kg of PSK, 1 KE/mouse of OK-432, 300 mu g/mouse of bestatin or 3mg/kg of levamisole was respectively injected intraperitoneally for four consecutive days before or after irradiation. The anti-tumor effect was evaluated by the changes of tumor volume and survival curves. When PSK, OK-432 or levamisole was administered after irradiation, tumor growth was decreased and sixty-day survival rate and survival curve were significantly elongated compared with the control group and the group to which PSK, OK-432 or levamisole administered before irradiation (p lt 0.01). As for bestatin, no remarkable difference was observed irrespective of the timing of administration. These results suggested that some immunomodulators showed different anti-tumor activity depending on its timing of administration in the combination with radiotherapy. Concerning PSK, OK-432 and levamisole, they seemed to be beneficial in combination with prior radiotherapy in the treatment of MM46 tumor. (author)