Monocytes/macrophages (M phi) have been implicated in the pathogenesis of lupus nephritis (LN), but the precise molecular mechanism of recruitment and activation of M phi in LN remains unclear. To clarify the involvement of chemotactic cytokines (chemokines) in those events, we measured levels of monocyte chemotactic and activating factor (MCAF, also termed monocyte chemoattractant protein-1, MCP-1) in urines and sera derived from 42 patients with LN. Both urinary and serum MCAF levels were significantly higher in patients with LN as compared with 22 healthy volunteers (10.3 +/- 3.2 vs. 1.0 +/- 0.1 pg/ml . creatinine, 212.2 +/- 75.8 vs. 66.1 +/- 15.5 pg/ml, respectively, P < 0.05, mean +/- SEM). Histological examination of renal lesions from 41 patients classified 19 as active according to the WHO-defined classes IIIb, IVb and IVc, and 22 as inactive by the WHO-defined classes I, II, IIIc, IVd and V. Urinary MCAF levels in the patients with active lesions were significantly higher than those with inactive lesions (20.3 +/- 6.4 vs. 1.7 +/- 0.3 pg/ml . creatinine, P < 0.01). Moreover, elevated urinary MCAF levels were dramatically decreased during steroid therapy-induced convalescence in 29 patients examined serially (13.9 +/- 4.5 vs. 5.3 +/- 1.7 pg/ml . creatinine, P < 0.001), whereas serum MCAF levels did not change significantly. Endothelial cells, renal epithelial cells and infiltrating mononuclear cells in the tubulointerstitial regions were MCAF-positive in immunohistochemical as well as in situ hybridization analysis. These observations suggest that MCAF is probably involved in the pathogenesis of LN with active lesions, possibly through the recruitment and activation of M phi, and that measurement of urinary MCAF levels may be a useful clinical tool for monitoring the disease activity of LN.