A radioimmunoassay for the determination of somatostatin-like immunoreactivity (SLI) in plasma has been developed using antibodies raised in rabbits against synthetic somatostatin conjugated to hemocyanin and 125I-Tyr1-somatostatin. The detection limit was 2 pg/ml. A combination of EDTA and aprotinin (0.15 M and 80 Kallikrein Inhibitor Units = KIU/ml) inhibited the tracer degradation, but did not eliminate it completely. In addition somatostatin immunoreactivity in unextracted plasma showed a strong dilution effect and the estimates depended on the antiserum used (e.g. a plasma sample (1 : 5) assayed with antiserum R 141 apparently contained 150 pg/ml, whereas antiserum K 5615 yielded a value of 300 pg/ml). When plasma was extracted with acid ethanol, the values obtained with the two different antisera were practically the same, recovery of somatostatin added to plasma was approx. 80% and dilution of specimens gave proportional readings. Consequently, extraction of plasma is regarded as obligatory to ensure valid results. Mean basal level was 26.5 +/- 1.2 pg/ml (mean +/- SEM). When plasma extracts were subjected to chromatography on Sephadex G-25 at neutral pH, approx. 30% of the SLI was recovered in the MW1600 fraction; whereas the major part of the remaining immunoreactivity (37-60%) eluted earlier in one fraction and the residual immunoreactivity in the void volume.