The elongation factor eEF1B involved in protein translation was found to contain two isoforms of the eEF1Bdelta subunit in sea urchin eggs. The eEF1Bdelta2 isoform differs from eEF1Bdelta1 by a specific insert of 26 amino acids. Both isoforms are co-expressed in the cell and likely originate from a unique gene. The feature appears universal in metazoans as judged from in silico analysis in EST-databanks. The eEF1B components were co-immunoprecipitated by specific eEF1Bdelta2 antibodies. Quantification of the proteins in immunoprecipitates and on immunoblots demonstrates that eEF1Bdelta1 and eEF1Bdelta2 proteins are present in two subsets of eEF1B complex. We discuss and propose a model for the different subsets of eEF1B complex concomitantly present in the cell.