The insulin-like growth factors (IGFs) are important regulators of growth and development in eutherian mammals. In this study we have analyzed circulating insulin-like growth factor binding proteins (IGFBPs) by Western ligand blotting (WLB) and neutral gel filtration, and hepatic IGFBP mRNA transcripts in the wallaby, Macropus eugenii, a marsupial in which the fetus is born at an immature stage compared to eutherian mammals. Plasma from male and female adults, lactating mothers, and pouch young contains an IGF binding species consisting of a 42- to 50-kDa doublet, 30, 28, 24, and approximately 200 kDa, as shown by WLB. This pattern of IGFBPs is very similar to that observed in human and sheep. Neutral gel filtration revealed IGF binding activity of molecular size 70-160 kDa. When electrophoresed under nonreducing, denaturing conditions, the 70- to 160-kDa IGF binding activity appeared as a 42- to 50-kDa doublet, indicating the presence of a high-molecular-weight circulating IGF binding complex. These properties are again analogous to those observed for eutherian IGFBPs. Northern analysis of total wallaby liver RNA detected transcripts of 1.8 kb for IGFBP-1 and 1.6 kb for IGFBP-2, the same as those observed in the sheep and rat. Transcripts of 3.2 kb were detected for IGFBP-4, larger than the major band of 2.6 kb observed for the sheep and rat. Transcripts representing wallaby IGFBP-3 mRNA were not detected using a rat IGFBP-3 cDNA probe. Circulating IGFBPs were analyzed in plasma from wallaby mothers and their pouch young. There were no changes in circulating IGFBPs in the wallaby mothers throughout lactation.(ABSTRACT TRUNCATED AT 250 WORDS)