We examined complement control proteins focusing on the role of modulating the complement activation in IgA nephropathy. Glomerular C4-binding protein (C4-bp) deposits were found in 60% of patients with IgA nephropathy, while C4 deposits were found in 30%. Glomerular deposits of beta 1H globulin (beta 1H) were found in 85% of patients with IgA nephropathy. The frequency of glomerular deposits of C4-bp tended to be higher in the group with deposits of various immunoglobulin types than in the group with deposits of IgA alone, and it increased parallel with the progression of glomerular histologic changes. The serum C4-bp level was higher in IgA nephropathy patients. No significant correlation was found between the serum level of C4-bp or beta 1H and the extent or distribution of tissue deposits of these complement control proteins. These results suggest that glomerular immune deposits in IgA nephropathy may be attributable to the activation not only of the alternative pathway but also of the classical pathway, the latter of which may take place locally in glomeruli with advanced histologic change or various immunoglobulin deposits in IgA nephropathy.