A spontaneously occurring IgM-mediated mesangioproliferative nephropathy was detected in 91% of tamarins and marmosets over 6 months old. Intraglomerular IgM deposits could be demonstrated in all animals, usually in a mesangial pattern and in some cases along the glomerular endothelia. The clinical signs were hematuria and proteinuria. The progressive disease remains silent for long periods, but is directly or indirectly related to the death of 10-20% of the affected animals. Morphological characteristics are those of mesangial hyperplasia accompanied by subacute to chronic tubulointerstitial inflammation. The callithrid IgM-nephropathy is compared to similar human immunocomplex nephropathies.