Affinity-purified antibodies prepared against the peridinin-chlorophyll a-protein (PCP) complex from the dinoflagellate Heterocapsa pygmaea were employed to study the immunological similarity of PCP among 28 dinoflagellate species, representing eight genera in four families. The anti-HpPCP antibodies cross-reacted with the subunits of PCP apoproteins from all dinoflagellates tested, but did not cross-react with pigment proteins from the chrysophyte Cricosphaera carterae, establishing its specificity for dinoflagellate pigment-protein complexes. Among the dinoflagellates, the PCP apoprotein occurs either as a monomer of about 35 kDa or as an apparent homodimer of about 15 kDa. In some instances, both subunit polypeptides are present simultaneously. The occurrence of different quaternary structures of the PCP apoprotein in different algae can serve as a taxonomic tool, when used in conjunction with other characters.