Gastrin releasing peptide(GRP)-like immunoreactivity in human plasma was measured using radioimmunoassay of neuromedin C (NMC) in 83 healthy and 58 diseased subjects. In the healthy group, the mean value of fasting GRP-like immunoreactivity was 2.1 +/- 1.4 (mean +/- SD) pmol/L. There was a slight positive correlation between the GRP-like immunoreactivity values and aging. Postprandial serial measurements demonstrated that GRP-like immunoreactivity showed no response to a significant elevation of serum gastrin concentration. The group with chronic renal failure on hemodialysis gave the highest value, 7.1 +/- 2.1 pmol/L (p less than 0.01). There were no statistical differences between the healthy controls and groups with peptic ulcer, liver cirrhosis, diabetes mellitus or carcinomas, although some cancer patients had a marked increase in GRP-like immunoreactivity value.