The relationship between the immune complex deposits of mesangial IgA nephropathy and the basement membrane components, type IV collagen and fibronectin, has been investigated by an indirect immunogold technique in four cases of mesangial IgA disease. Using paraformaldehyde-fixed, Lowicryl K4M resin-embedded kidney, IgA, IgM and C3 were localized in the mesangial electron-dense deposits with 10 and 20 nm gold-labelled secondary antibodies. In the same glomeruli, type IV collagen and fibronectin were rarely present within the electron-dense deposits, although both were distributed throughout the remainder of the mesangial matrix with the exception of the subepithelial regions. These two components were also present within the glomerular basement membrane and localized mainly on the endothelial aspect. A similar distribution of the basement membrane components was seen in a control kidney processed in the same way. This technique gives reproducible results and has demonstrated for the first time the relationship between the mesangial immune complex deposits of mesangial IgA nephropathy and the basement membrane components of the matrix in which they are found.