The phosphotriesterase in chicken serum that hydrolyses O-hexyl O-2,5-dichlorophenyl phosphoramidate (HDCP) was purified in three chromatographic steps. The activity copurified to apparent homogeneity with albumin monitoring by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS/ PAGE) and by SDS-capillary electrophoresis in the purified fractions. Commercial chicken serum albumin was further purified and the phosphotriesterase activity remained associated with albumin. Capillary electrophoresis established a molecular weight of 59 +/- 4 kDa for both purified proteins (chicken serum and commercial chicken serum albumin). The purified samples were assayed for hydrolytic activity against several carboxylesters, organophosphates and phosphoramidates. From carboxylesters, only p-nitrophenylbutyrate (p-NPB) hydrolysing activity was found to copurify with the phosphotriesterase. The purified human, chicken, rabbit and bovine serum albumins and recombinant human serum albumin obtained from commercial sources hydrolysed HDCP and p-NPB. Serum albumin also hydrolysed O-butyl O-2,5-dichlorophenyl phosphoramidate, O-ethyl O-2,5-dichlorophenyl phosphoramidate and O-2,5-dichlorophenyl ethylphosphonoamidate but not other organophosphates and phosphoramidates.