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Radioimmunoassays (RIAs) of insulin-like growth factor-I (IGF-I) in nonmammals are complicated by two problems: decreased specificity due to heterologous assays and assay interference from IGF binding proteins. We have developed and validated an RIA fo the detection of IGF-1 in reptilian plasma that circumvents these problems. Biochemical validations show parallelism between standard curves, plasma dilutions, and internal standards. Further, biological validations indicate that (1) plasma IGF-I levels are significantly reduced in nonreproductive, female American alligators (Alligator mississippiensis) with high plasma estradiol-17β concentrations and (2) plasma IGF-I concentrations are elevated in juvenile Trachemys scripta elegans turtles fed higher quality diets (defined by protein and energy content). These results provide impelus for future studies of IGF-I in reptiles. Copyright 1995, 1999 Academic Press