Immunoprecipitation of biosynthetically labelled interleukin-4 (IL-4) secreted by activated D10 cells yielded three bands after separation under non-reducing conditions by electrophoresis through gradient SDS polyacrylamide gels. This triplet consists of a closely spaced doublet at 23 and 22 kDa (23/22 kDa), and a third band at 18.5 kDa (19 kDa). The 23/22 kDa doublet was converted to the 19 kDa form by enzymatic removal of the N-linked sugars, indicating that the two glycoforms were derived from the 19-kDa core polypeptide. Under reducing conditions, the 19-kDa polypeptide migrated at 14.5 kDa, consistent with the size predicted from the complementary DNA (cDNA). Under non-reducing conditions, IL-4 retained biological activity after electrophoresis and transfer to nitrocellulose. Applying a biological assay, proliferation of the NK cell line, to fractionated nitrocellulose replicas, we found that IL-4 activity was detected over a relatively broad range of molecular weights, reflecting the multiple bands found by immunoprecipitation. This was true not only for IL-4 produced by D10 cells but also for splenic cells activated in vitro. All of the immunoprecipitated IL-4 species were active in inducing proliferation of the NK cell line. However, when the D10 cell line was used to detect IL-4, the 19-kDa species was significantly more active than the higher molecular weight species. These results suggest that different forms of IL-4 may have different functional properties.