By immunizing rabbits – tolerant against the bulk of normal human serum proteins – with highly purified non-suppressible insulin-like activity (NSILA-S), an anti-serum was obtained which made possible the development of a double-antibody radioimmunoassay. Its sensitivity is about 30 pg NSILA-S per tube or 150 pg NSILA-S/ml. The specificity exceeds that of the bioassay used for comparison which is based on the stimulation by NSILA-S of <sup>125</sup>IUDR incorporation into chicken fibroblasts in culture. The radioimmunoassay is sufficiently sensitive and specific to allow direct NSILA-S measurement in serum or plasma samples of humans and experimental animals. In human plasma samples NSILA-S levels, varying between less than 0.15 and 25 ng/ml, were found to have an average of about 4 ng/ml. In rats higher levels were observed with a mean of 7.7 ng/ml in 4-week-old animals, increasing to about 60 ng/ml in 6-month-old rats. In fasting rats the NSILA-S plasma level is reduced. In acid-treated samples of plasma considerably higher NSILA-S amounts are found.