The expression vector lambda gt11Amp3 has been used to construct a cDNA library from rat liver polyadenylated RNA. Clones expressing antigenic determinants for rat albumin, transferrin, transthyretin, apolipoprotein E and apolipoprotein AII have been identified. Albumin clones containing cDNA inserts ranging from 0.9 kb to 1.9 kb were further identified by restriction mapping and nucleic acid sequencing. The largest insert contained the entire coding sequence for albumin. Characterization of the expressed proteins by acrylamide gel electrophoresis followed by immunological detection indicated that the proteins were produced as hybrids linked to the bacterial beta-galactosidase. A cDNA library for human liver polyadenylated RNA has also been constructed. Clones expressing antigenic determinants for human serum albumin, transferrin and apolipoproteins AI, AII, AIV and E have been isolated and their identity established by nucleotide sequencing and restriction mapping. Both rat and human serum protein cDNA clones are currently being used to study the tissue specific expression of serum proteins and for the isolation and characterization of the corresponding genes.