In order to study the relationship between obese protein and obesity. A radioimmunoassay for human plasma obese protein (OP) was established using human OP (57-92) and its specific antiserum. We observed the distribution and the characteristics. Of OP in normal human and mouse tissue and changes of plasma OP in obese patients. We also observed the tissue distribution after intravenous injection of 125I-labeled OP fragments (116-167) and (93-105) in rats. The results were as follows. The contents of OP were much higher in brain tissue than those in abdominal adipose tissue; levels of OP in liver and in skeletal muscle tissue were zero; the contents of abdominal adipose tissue were higher in female mice than those in male mice; the contents of plasma OP in normal human beings were 194.3 +/- 17.7 ng/L, while those in mice were 2257.8 +/- 171.9 ng/L. It was also found that the administered OP fragments were widely distributed in various tissue and organs including the brain. Kidney was the richest in the OP fragments; liver and lung ranked second. The half time of the OP fragments in plasma clearance was about 4.5 min. The contents of OP in obese adults and in obese children were much lower than those in normal control group. There was significant negative correlation between OP levels and body mass index, serum concentration of glucose, cholesterol and triglycerides. So the present study indicates the important relationship between the changes of OP contents and the pathogenesis of obesity.