Gel-permeation chromatography of serum on Sephacryl S-300 at pH 7.4 has shown that NSILA was detected over a range of MW 50,000-400,000 with a peak at about MW 200,000. When fractions from the above chromatography were rechromatographed on Sephadex G-75 at pH 2.4 major amounts of acid-stable NSILA were found in a fraction of MW 200,000-600,000 (77% of the fraction NSILA or 28% of total serum NSILA). Further evidence was obtained for the presence of an active acid-dissociable complex in serum. This was present in both the MW 100,000-200,000 and 35,000-100,000 fractions and corresponded to 37% of total serum NSILA. Con-A Sepharose affinity chromatography of the serum fractions from Sephacryl S-300 chromatography, followed by Sephadex G-75 chromatography under acid conditions, showed that the acid-stable complex was consistently found in weakly bound materials. The active acid-dissociable complex was found in the bound fractions, especially in the Sephacryl S-300 pool of MW 35,000-100,000. Low MW NSILA (less than 15,000) was also released on acid treatment from an otherwise inactive high MW complex(es) of MW 35,000-600,000. This complex was not bound by Con-A Sepharose.