This investigation describes the presence of insulin-like growth factor-I (IGF-I) binding proteins in chicken serum. Whole blood was collected from broiler chickens of 7-9 weeks of age and analysed for binding proteins after gel permeation chromatography under both neutral and acidic conditions, and by polyacrylamide gel electrophoresis in the presence of 12.5% sodium dodecyl sulphate (SDS-PAGE). When serum was chromatographed under neutral conditions, about 70% of the IGF-I immunoreactivity was associated with a large protein complex (Mr = 150,000) and 20-25% was associated with an intermediate-sized protein complex (Mr = 45,000). Up to 6% of the serum IGF-I immunoreactivity was eluted in a fraction which corresponded to an Mr of about 7500 and was presumably free IGF-I. Chromatography under acidic conditions dissociated the IGF-I/protein complexes and revealed the presence of an acid-stable binding protein (Mr = 50,000-60,000). After analysis of serum by SDS-PAGE, three monomeric IGF-I binding proteins (Mr = 28,800, 33,200 and 40,700) were detected. The largest monomer (Mr = 40,700) is probably the binding protein component of the intermediate-sized IGF-I/protein complex. The relationship between the other monomers and both the large IGF-I/protein complex and the acid-stable binding protein is not known. Although the pattern of binding proteins in chicken serum is similar to that observed in mammals, a major difference is the presence of up to 6% of the serum IGF-I immunoreactivity in an unbound form.