19 nor, 10 keto, 25-hydroxyvitamin D3 (19/10-25OHD3) is a metabolite of 25-OHD3 produced in vitro by various phagocytes including normal human blood monocytes and transformed cell lines, U937 and HL-60. We recently reported that 19/10-25OHD3, induced differentiation of U937 cells. In these studies, 19/10-25OHD3 alone produced no detectable effect on the growth rates, surface adherence, and oxidative metabolism of U937 and HL-60 cells. When combined with lymphocyte-conditioned medium (LCM), 19/10-25OHD3 reduced proliferation, increased surface adherence and stimulated luminol-dependent luminescence (LDL) of the U937 cells. In contrast, the combination of 19/10-25OHD3 and LCM had no effect on the growth of HL-60 cells but did increase the surface adherence and the expression of a complement receptor component. 19/10-25OHD3 competed for tritium-labeled 1,25(OH)2D3 binding to receptors extracted from cultured human skin fibroblasts. This displacement capacity was 600 times weaker than that of unlabeled 1,25(OH)2D3. Incubation of human skin fibroblasts for 24 hr with 19/10-25OHD3 induced 25OHD3-24-hydroxylase activity in the fibroblasts. The inductive potency of 19/10-25OHD3 was 1/50 that of 1,25(OH)2D3. These results demonstrate bioactivity of 19/10-25OHD3 in several systems. At least one of these responses, the induction of 25OHD3-24-hydroxylase, is a receptor-mediated event. Some of the other responses may be independent of the cellular receptor for 1,25(OH)2D3. Interestingly, the potency of 19/10-25OHD3 was highest in the receptor-mediated response (1:50) and lower in the other parameters, ranging from 1:100 to 1:600 compared to 1,25(OH)2D3. This range of bioactivity in phagocytes and fibroblasts is presently explained.