Des-(1-3) insulin-like growth factor-I (IGF-I) is an IGF analogue lacking aminoacid 1 to 3 which displays reduced binding to insulin-like growth factor binding protein-1 (IGFBP-1). A greater inhibition of immunoreactive IGFBP-1 was obtained with des-(1-3) IGF-I (10 ng/ml) in Hep G2 medium when incubated in Eagle's Modified Essential Medium (EMEM) without phenolred compared to EMEM with phenolred; EMEM without phenolred was chosen for further experiments. Des-(1-3)IGF-I decreases dose dependently the concentration of IGFBP-1, with a maximal effect at 3–10 μg/l when incubated for 24 h; 10 μg/l of des-(1-3)IGF-I caused a small but significant inhibition of IGFBP-1 after 8 h incubation and this inhibition was 41% and 33% of controls after 14 and 19 h incubation. The relative potencies at 16 h of incubation of IGF-I and insulin in suppressing IGFBP-1 in comparison to des-(1-3)IGF-I were 0.41 (0.25–0.78) and 0.08 (0.01–0.26), respectively. A dose-dependent decrease of IGFBP-1 mRNA to 30% of control was observed after 4 h incubation with 0.1–10 μg/l des-(1-3)IGF-I. Changes of glucose concentration (0–20 mmol/l) in the medium did not affect the IGFBP-1 concentration in the medium. In summary: Des-(1-3)IGF-I was tenfold more potent than insulin, and threefold more potent than IGF-I in decreasing IGFBP-1 concentration in medium conditioned by Hep G2 cells.