Hyperimmunized rabbit anti-rat apolipoprotein antibodies showed reactivity to proteins in Saccharomyces cerevisiae. Antiapolipoproteins A1, B and E reacted with proteins in both a crude extract and a lipid enriched fraction. Protein reactivity was dependent on antisera dilution. Furthermore, the different antiapolipoproteins tested reacted with individually distinct yeast proteins, possibly suggesting the presence of a variety of yeast apolipoproteins with distinct cellular functions as is the case with mammalian apolipoproteins. The specificity of the antibody was directed toward the yeast protein and not a lipid moiety.