We studied some of the biochemical and immunochemical properties of a major apolipoprotein in isolated pulmonary surfactant from dog and rat lungs. These apolipoproteins were purified by DEAE-cellulose chromatography in buffers containing Triton X-100. Purity of the apolipoproteins was assessed by both fused rocket and crossed immunoelectrophoreses. In addition, the apolipoproteins showed one band with an apparent molecular weight of 72 000-73 000 on SDS-polyacrylamide gel electrophoresis. These proteins are composed of two polypeptide chains of 36 000 daltons. When subjected to isoelectric focusing, the major component of the apolipoprotein had an isoelectric point of about 4.4, with very minor components near 4.6. Even though the apolipoproteins of both species had very similar amino acid compositions, including a relatively high glycine content, no immunologic cross-reactivity was observed. Rocket immunoelectrophoretic analysis of several preparations of dog and rat surfactant using the respective purified apolipoproteins as standards indicated that the apolipoprotein constituted 56.9% +/- 4.6. (S.D., n = 3) and 42.1% +/- 2.1 (S.D., n = 2) of the total protein in dog and rat surfactant, respectively.