Poly-beta-hydroxybutyrate (PHB) is an amphiphilic lipid that has been found to be a ubiquitous component of the cellular membranes of bacteria, plants and animals. The distribution of PHB in human plasma was investigated using chemical and immunological methods. PHB concentrations proved highly variable; in a random group of 24 blood donors, total plasma PHB ranged from 0.60 to 18.2 mg/l, with a mean of 3.5 mg/l. In plasma separated by density gradient ultracentrifugation, lipoproteins carried 20-30% of total plasma PHB; 6-14% in the very low density lipoproteins (VLDL), 8-16% in the low density lipoproteins (LDL), and less than 3% in the high density lipoproteins (HDL). The majority of plasma PHB (70-80%) was found in protein fractions of density greater than 1.22 g/ml. Western blot analysis of the high density fractions with anti-PHB F(ab')2 identified albumin as the major PHB-binding protein. The affinity of albumin for PHB was confirmed by in vitro studies which demonstrated transfer of 14C-PHB from chloroform into aqueous solutions of human and bovine serum albumins. PHB was less tightly bound to LDL than to other plasma components; the polymer could be isolated from LDL by extraction with chloroform, or by digestion with alkaline hypochlorite, but it could not similarly be recovered from VLDL or albumin. PHB in the LDL correlated positively with total plasma cholesterol and LDL cholesterol, and negatively with HDL cholesterol. The wide concentration range of PHB in plasma, its presence in VLDL and LDL and absence in HDL, coupled with its physical properties, suggest it may have important physiological effects.