Administration of puromycin aminonucleoside (PAN) to rats induces acute nephrosis with hyperlipidemia, and, in some experimental conditions, it results in chronic focal glomerulosclerosis. In this study, we examined the cytotoxicity of serum from rats with PAN-induced nephrosis, since hypercholesterolemia is considered to cause injury to vascular walls in atherosclerosis, the mechanism of which is analogous to that of glomerulosclerosis. About half of the tested sera from nephrotic rats (9 out of 17) were cytotoxic to cultured aortic endothelial cells. The toxic substance(s) was heat-stable and was extracted in the lipid fraction. Serum levels of triglyceride and cholesterol were markedly higher in the group of rats with cytotoxic serum than in the group with noncytotoxic serum. No cytotoxicity was associated with sera from control rats or the corresponding lipid fractions. Cytotoxic sera were also effective against cultured glomerular epithelial and mesangial cells. These results indicate that cytotoxic lipid is produced in rats with PAN nephrosis and the results raise the possibility that the cytotoxic lipid in nephrotic serum might contribute to lipid-mediated glomerular injury which may induce glomerulosclerosis at a subsequent stage.