Plasma factors influencing PGI2-like activity in 19 patients with diabetes mellitus /Dm/ and 17 controls were studied through a comparison with the signs of retinal and glomerular angiopathy. The plasma PGI2 supporting activity /PSA/ was lower in 15 Dm cases than in the controls. Inhibitory activity against PGI2 production was detected in 6 patients. In the cases of more serious retinopathy associated with glomerulopathy, a significantly lower level of PSA was observed than in patients with mild retinopathy without glomerular diseases. The plasma concentrations of total and LDL-cholesterol were significantly higher, while the level of HDL-cholesterol was lower than in the controls. There was a positive correlation between PSA and HDL-cholesterol values and a negative correlation between PSA and LDL-cholesterol levels, which relates to an inhibitory effect of LDL and a protective role of HDL in PGI2 synthesis.