Using radioimmunoassay, immunoreactive SP (iSP) has been measured in the plasma of 162 hospital patients with various disorders and in the plasma of 67 pregnant women. In pregnancy, iSP was undetectable in 40% women as compared to 12% in other hospital patients. The mean iSP plasma level in pregnancy was 37.8 ± 4.6 (SEM) as compared to 77.1 ± 4.9 (SEM) pg/ml in other hospital patients. These results support earlier observations based on bioassay, suggesting that the blood of pregnant women contains higher concentrations of a SP-inactivating factor. Of the hospital patients, elevated levels of iSP were found in patients with chronic leukaemia, in one patient with a basaloid carcinoma of the anus, and in one patient with toxic liver damage and pancreatic insufficiency. No correlation was found between thyroid function and iSP plasma levels. ISP plasma levels in various gastrointestinal disorders were similar to those found in normal subjects.