This paper deals with the immunosuppressive effects of 1,25-dihydroxy vitamin D3 (1,25(OH)2, D3), a metabolic product which acts in vivo as the active form vitamin D3. Dose and time of exposure-dependent suppressive effects were demonstrated in vitro on PHA blastogenesis of mononuclear cells, IL-2 production of T cells and IL-1 production of macrophages when these human immunocytes were treated with 1,25(OH)2D3. Addition of exogenous IL-1 and IL-2 partially abrogated the suppressive effects of 1,25(OH)2 D3 on IL-2 production and PHA blastogenesis, respectively. Thus, it is suggested that inhibition of PHA blastogenesis results from the reduced production of such growth factors in this IL-1 -IL-2 cascade system. PWM-induced immunoglobulin (Ig) synthesis of B cells was also markedly inhibited by 1,25(OH)2 D3, which was caused not only by direct suppression toward B cells but also by indirect suppression due to reduced helper factor from T cells when these cells were treated with 1,25(OH)2 D3. It is proposed that 1,25(OH)2D3 may act as one of the immunoregulatory factors in vivo, in addition to its well-known action in calcium metabolism.