This study was performed to investigate the effects of transforming growth factor-β1 (TGF-β1) latency-associated peptide (LAP) and betaglycan on TGF-β1 activity, and on the glucose-induced overproduction of fibronectin in cultured human mesangial cells (MCs). We found that recombinant LAP and recombinant soluble betaglycan decrease the active form of TGF-β1, measured by ELISA, in a dose-dependent manner in a cell-free system. The effective dosages of LAP and soluble betaglycan for a 50% reduction were approximately 20- and 75-fold of the TGF-β1 concentration, respectively. The active form of TGF-β1 in the media secreted from MCs was significantly (p < 0.01) reduced by the addition of 10 nmol/l LAP and 10 nmol/l soluble betaglycan with no significant change in total (active + latent) TGF-β1. Recombinant LAP and soluble betaglycan also inhibited a recombinant TGF-β1-stimulated increase in fibronectin production in MCs. Furthermore, the glucose-induced increase in fibronectin secreted from MCs was significantly (p < 0.01) suppressed by concomitant incubation with LAP or soluble betaglycan, while these agents had no effect on fibronectin production under physiological glucose concentrations. These results indicate that recombinant LAP and soluble betaglycan suppress the glucose-induced overproduction of fibronectin presumably via inhibition of TGF-β1 activity in MCs. Further in vivo studies are needed to define the possible beneficial effects of these agents in diabetic nephropathy.