The allophycocyanin core of hemiellipsoidal phycobilisomes from the red alga, Porphyridium cruentum, was isolated by chromatography on hydroxylapatite and subsequent density gradient centrifugation. Electron microscopy of negatively stained core complexes revealed a tricylindrical structure with a width of 21 to 23 nm in face view and a depth of 12 to 14 nm in side view. Fluorescence emission spectra of these complexes were similar to those of whole phycobilisomes confirming the presence of the two "terminal energy acceptors" allophycocyanin B (APB) and the high molecular linker polypeptide LCM. The polypeptide composition analyzed by SDS-PAGE showed the "anchor" polypeptide LCM, alpha AP, alpha APB, beta AP subunits, a low molecular weight linker with M(r) 13,500 and a blue-colored polypeptide of M(r) 19,800. A complex containing APB could be isolated from a "trimeric" allophycocyanin fraction of the density gradient by agarose gel electrophoresis in the presence of ampholytes. This complex shows a polypeptide composition of (alpha APB alpha AP2 beta AP3).L13.5C and contributes to the core with 30 to 35% of total "trimers". In comparison to the allophycocyanin cores from hemidiscoidal phycobilisomes of cyanobacteria and red algae, all data concerning the core of the hemiellipsoidal phycobilisomes from Porphyridium cruentum strongly suggest that there is no increase in size but an increase in its APB content. A model comprising the allophycocyanin core and the membrane integral photosystem II particles presents the structural and possible functional consequences of the results.