The interaction between prealbumin and apo or holo retinol-binding proteins has been studied by affinity chromatography, gel filtration and two-phase partition. At physiological ionic strength apo and holo retinol-binding protein form 1:1 molar complexes with prealbumin. Mean dissociation constants for the prealbumin compex with apo retinol-binding protein and holo retinol-binding protein with all-trans retinol, retinoic acid, retinal and retinyl acetate were calculated from the partition data as 0.33 +/- 0.11 x 10(-6) M and 0.075 +/- 0.015 x 10(-6) M respectively (mean +/- S.E.M.). The difference was statistically significant. Quantitative data on the amount of retinol, retinol-binding protein and prealbumin in plasma and urine were in good agreement with the ratio of the dissociation constants for the complexes of apo and holo retinol-binding proteins with prealbumin as determined in the partition experiment. The magnitude of the dissociation constants was compatible with previously published data on the turnover of retinol-binding protein.