Supernatant factor from peripheral blood mononuclear cell (PBMC) cultures of idiopathic minimal lesion nephrotic syndrome (IMLNS) patients in relapse induces in vivo albuminuria and increases 35 sulfate uptake by glomerular basement membrane (GBM) in rats. The purpose of this study was to evaluate the effect of anti-interleukin 8 (IL8) neutralizing antibody on the effects induced by the supernatant factor. Supernatant factor collected from six cultures of PBMC from IMLNS patients in relapse were combined and aliquoted into two samples. Anti-IL8 neutralizing antibody (750 ng) was added to one. Supernatant factor or supernatant factor and anti-IL8 antibody were infused for 5 days into the left renal artery of Wistar rats using an osmotic pump. On the last day of infusion, rats were injected with 35 sulfate (1.0 mCi/200 g) intraperitoneally and killed after 8 h. Glomeruli were isolated and GBM obtained. There was a significant increase in 35 sulfate uptake of the infused kidney (169+52 cpm/mg dry glomerular weight, mean + SEM) compared with the uptake seen in the contralateral kidney (116 + 41, P 50.05) when the supernatant factor was infused alone. No significant differences in 35 sulfate incorporation were seen between infused kidney (173 + 5) and contralateral kidney (190 + 49) when supernatant factor and anti-IL8 antibody were administered. A significant increase in albuminuria was seen on the last day of infusion (0.43 + 0.11 albumin/ creatinine ratio, mean + SEM) compared with the ratio prior to infusion of the supernatant factor alone (0.18 + 0.03, P 50.05). No significant differences in urinary albumin/creatinine ratios prior to and on the 5th day of infusion were seen when the supernatant factor was administered with anti-IL8 antibody. Supernatant factor effects were abolished by the addition of anti-IL8 neutralizing antibody, suggesting that the described effects are mediated by IL8.