Binding affinities (Ka) for thyroxine (T4) by blood plasma and purified plasma proteins from two turtles, the slider (Trachemys scripta) and snapper (Chelydra serpentina), were compared with those of a human using equilibrium dialysis. The purified T4 binding protein (TBP) from T. scripta had a high affinity that was lower by about fivefold than that of human TBG (3.2 x 10(9) vs 1.7 x 10(10) M-1). The affinity of T4 binding by TBP was similar to that determined for whole plasma from this species. T. scripta plasma stripped of TBP by affinity chromatography showed a greatly reduced Ka (2.8 x 10(5) M-1) similar to that of albumin purified from the same species (3.6 x 10(5) M-1), as well as a marked increase in free T4 (9.7%) as compared to whole turtle plasma (0.13%). The T4 binding of snapping turtle plasma (Ka = 5 x 10(5) and free T4 = 2.7%) was similar to that of the TBP-stripped plasma and albumin. Binding affinities for the two turtle and human albumins were similar. Because of low concentrations of albumin in the turtle blood (estimated at ca. 10 mg/ml based on binding), TBP probably accounts for a greater proportion of T4 binding in the slider turtle than does TBG in the human; e.g., when plasma T4 and TBP are elevated in T. scripta, > 98% of bound T4 would be associated with TBP. When both TBP and T4 are depressed (e.g., as in hatchlings), free T4 may actually be higher than in conditions when total T4 is elevated.(ABSTRACT TRUNCATED AT 250 WORDS)