IGF binding protein-3 (IGFBP-3), the major serum carrier of IGF-I and IGF-II in adults, circulates predominantly in a ternary complex with IGF and the acid-labile subunit (ALS). The affinities of IGF and ALS binding have not previously been determined under conditions of temperature, ionic strength and pH which approximate those in the circulation. IGF-I and -II binding was optimal at pH 4.0-5.5, and relatively poor at pH 7.4. The addition of 0.1 mol/l NaCl to 50 mmol/l phosphate buffer increased the affinity of IGF-II for IGFBP-3 by 2-fold, and of IGF-I by 4-fold. ALS binding peaked at pH 5.5-6.0, and was markedly reduced at pH 7.4, where only approximately 0.4 mol ALS was bound per mol IGFBP-3, with greatly reduced affinity. ALS affinity was further reduced at 37 degrees C compared to 22 degrees C, and in the presence of 0.1 mol/l NaCl compared to its absence. Under 'physiological' conditions, the affinities of ALS binding to IGF-I-IGFBP-3 and IGF-II-IGFBP-3 complexes (2.5 x 10(8) l/mol and 5.8 x 10(7) l/mol, respectively) are 300-fold and 2000-fold lower than the constants for the formation of the corresponding binary complexes. Thus, ALS binding to IGFBP-3 complexes is much weaker than previously recognised, emphasising the importance of ALS dissociation as a controlling factor in the regulation of IGF bioavailability.