The specific binding sites for growth hormone was recognized on both thymic lymphocytes (thymocytes) and thymus epithelial cells. The somatotropic action of GH is considered to be mediated mainly by insulin-like growth factor-1 (IGF-1) and partially by GH itself. In this study, effect of GH and IGF-1 on DNA synthetic activity of thymocytes was examined. By 48-hrs. culture with IGF-1 and 24-hrs. 3H-TdR pulse labeling, significant enhancement of DNA synthetic activity of thymocytes was detected, while the enhancement in the culture with GH was very weak. It is considered that IGF-1 acts on the cells in G0/G1 phase in cell cycle. The effect of IGF-1 on thymocyte proliferation was examined by using the thymocytes incubated 12 hrs. before IGF-1 stimulation and 3-hrs. 3H-TdR pulse labeling. The optimal condition to induce thymocyte proliferation was 15-hrs. culture with 380 ng/ml of IGF-1. Furthermore, 10 ng/ml or higher concentration of GH significantly increases IGF-1 release from TECs in confluent state. These results suggest that GH indirectly participates in thymus growth by increasing IGF-1 release from TECs, which enhances thymocyte proliferation.