IGF binding protein-5 (IGFBP-5) forms ternary complexes with IGFs and the acid-labile subunit (ALS) in vitro, but these complexes have not been demonstrated in the circulation. To examine the molecular distribution of circulating IGFBP-5 we developed an RIA with high specificity for IGFBP-5 among the IGFBPs, but wide cross-reactivity among primate and nonprimate species. The mean serum IGFBP-5 level (+/-SD) was 208 +/- 73 ng/ml in healthy men and 206 +/- 67 ng/ml in nonpregnant women, decreasing to 114 +/- 38 ng/ml in pregnancy. Approximately 55% of immunoreactive IGFBP-5 was associated with ternary complexes in nonpregnant adults, whereas only 35% was in these complexes in pregnancy serum. After transient acidification, all immunoreactive IGFBP-5 corresponded in size to free or binary-complexed protein. Serum IGFBP-5 levels were significantly associated with ALS levels (r = 0.478; P = 0.008), but the association was less than that between IGFBP-3 and ALS (r = 0.743; P < 0.001), reflecting the lower percentage of IGFBP-5 complexed with ALS. As free or binary complexed IGFBP-5 is a relatively high proportion of the total, we speculate that, alone or as a carrier of IGFs, IGFBP-5 might have preferential access to the tissues, where it could act to stimulate growth.