Bone cells in culture produce and respond to growth factors, suggesting that local as well as systemic factors regulate bone volume. Previous studies have shown that IGF-I is the major mitogen produced by mouse bone cells and that its production is regulated by systemic agents such as PTH and estrogen. Because IGF-II and transforming growth factor β1 have been shown, respectively, to increase and decrease MC3T3-E1 cell proliferation, we tested the hypothesis that these two growth factors modulate the production of IGF-I in this cell line. In order to eliminate artifacts owing to IGF binding proteins, conditioned media samples were pretreated with IGF-II before measurement of IGF-I by RIA. After 24 h treatment at a density of 2.5× 104 cells/cm2, IGF-II (10 μg/l) induced a 2.2-fold increase compared with untreated control (9.5±1.5 vs 4.2±0.44 pg/μg protein, p<0.001), whereas transforming growth factor β1 (1 μg/l) caused a 66% decrease in IGF-I production (1.5±0.3 vs 4.2±0.44 pg/μg protein, p<0.001). Both IGF-II and transforming growth factor β1 regulated IGF-I production in a dose-, time- and cell density-dependent manner. The lowest effective doses for IGF-II and transforming growth factor β1 were 1 and 0.01 μg/l, respectively. These results support a role for IGF-II and transforming growth factor β1 as potent modulators of IGF-I secretion in mouse bone cells. Furthermore, regulation of IGF-I production in bone cells by IGF-II and transforming growth factor β1 in an autocrine/paracrine manner could represent a component part of the mechanism whereby the skeleton locally adapts in reponse to external stimuli.