1. The isolation and purification of a putative apolipoprotein B-100 in the plasma of the freshwater turtle Chrysemys picta is described. 2. The protein was purified through differential ultracentrifugation and subsequent Sepharose 6B column chromatography. 3. The molecular weight of the protein determined by electrophoresis was approximately 350 kDa. 4. An antibody to chicken apolipoprotein B-100 specifically recognizes this 350 kDa protein in Western blots, suggesting its identity with apolipoprotein B-100. 5. An antibody to the putative Chrysemys apolipoprotein B-100-like protein was developed and used in an ELISA to quantitate protein levels in plasma. 6. Acute estrogen treatment increased levels of apolipoprotein B-100 (7.64 +/- 0.79 mg/ml plasma) over that of control animals (5.07 +/- 1.74 mg/ml plasma). 7. In contrast, chronic estrogen treatment reduced apolipoprotein B-100 significantly to 2.94 +/- 0.53 mg/ml plasma (P < 0.05).