Serum PG I levels were measured by an enzyme linked immuno sorbent assay (ELISA) in 567 blood donors and 171 patients from a routine gastroscopy program to study the relationship between PG I phenotypes and serum levels. In normal subjects no association was found between PG I phenotypes and serum levels. A low serum PG I level in patients was associated with phenotypes characterized by intense PG I fraction 5. This probably reflects the high frequency of these phenotypes in patients with atrophic gastritis or gastric cancer. The mean (+/- SD) serum PG I level was 45.8 +/- 17.7 micrograms/1 in control subjects. Serum PG I levels were lower in females (41.4 +/- 17.5) than in males (47.3 +/- 17.7) and increased with advancing age up to 65 years.