The ultrastructural and histochemical alterations during different stages of spontaneously occurring IgM nephropathy in Callitrichidae were determined. Callitrichid IgM nephropathy was classified according to the light microscopic sequence of the mesangial lesions in the individual glomeruli. In minimal disease and axial proliferation mesangial cells and mesangial matrix were symmetrically increased, whereas in panmesangial disease matrix deposition exceeded the cellular responses. However, ultrastructural matrix changes such as deposition of electron-dense particles, dilatation of the matrix channel system, and slightly increased collagen fiber expression were present also in minimal disease and axial proliferation. Histochemically collagen I and III phenotypes could only be verified in glomerular sclerosis, at that time accompanied by diminution of collagen IV. The mesangial cellular lesions, in contrast, were very limited. The numerical increase in mesangial cells was associated with increased bleb formation and increased heterolysosome contents, whereas the amount of microfilaments was never increased.