To confirm that the soluble factors produced by peripheral blood mononuclear cells (PBMC) from children with minimal change nephrotic syndrome (MCNS) enhance thromboxane (TX) A2 synthesis, we studied the urinary albumin and TXA2 metabolite excretions of rats intravenously injected with PBMC culture supernatant (PCS). Furthermore, we studied which does mainly effect on urinary albumin excretion, TXA2 metabolism in kidney or in platelet. In this study, the urinary albumin, TXB2, and 11-dehydro-TXB2 excretions of rats injected with PCS from MCNS children were remarkably increased, but this change was not observed in rats injected with PCS from children with MCNS in remission, glomerulonephritis, or healthy controls. The urinary albumin excretion of rats administered with acetylsalitylic acid before injection of PCS from MCNS children did not increase. We concluded that the soluble factors produced by PBMC from MCNS children increase urinary albumin excretion, and that the TXA2 metabolism is closely related to this effect.