The effects of Ganoderma lucidum (Gl) and Krestin (PSK) extracts on cellular immunocompetence, leukocyte counts and differential count in gamma-irradiated mice were investigated in this study. ICR strain male mice were used and randomly divided into five groups. Group A is normal control. Group B, the experimental control, was treated with Gl. Group C, the radiation treatment control, was treated with whole body exposure to 4 Gy gamma-irradiation (RT). Group D was treated with RT and Gl. Group E was treated with RT and PSK. The dosage of Gl was 400 mg/day/kg body weight and PSK was 500 mg/day/kg body weight. After irradiation, six mice from each group were sacrificed on day 7 and the other six on day 28. Cellular immunocompetence was measured by means of 3H-thymidine incorporation with splenic cells stimulated through mitogens such as PHA, Con A and LPS. The results revealed that relative splenic weight in Groups D and E were higher than group C on day 28 after gamma-irradiation, Group D was the highest in all the experimental groups. Leukocyte counts were decreased significantly in Groups D and E on day 7, the former was a little higher than the latter. Gl administration showed an increase in the leukocyte count in Group D on day 28. The blastogenic response of splenocytes to PHA and Con A in groups D and E were higher than in Group C on days 7 and 28. We suggested that Gl and PSK were effective in enhancing the recovery of cellular immunocompetence from gamma-ray irradiation.