Two very closely related human E2 ubiquitin conjugating enzymes, UbcH5B and UbcH5C, have been identified. These enzymes are products of distinct genes and are 88-89% identical in amino acid sequence to the recently described human E2, UbcH5 (now designated UbcH5A). UbcH5A-C are homologous to a family of five ubiquitin conjugating enzymes from Arabidopsis thaliana, AtUBC8-12. They are also closely related to Saccharomyces cerevisiae ScUBC4 and ScUBC5, which are involved in the stress response, and play a central role in the targeting of short-lived regulatory proteins for degradation, mRNAs encoding UbcH5A-C were co-expressed in all cell lines and tissues evaluated, with UbcH5C transcripts generally expressed at the highest levels. Analysis of Southern blots suggests that there are likely to be other related members of this family. Both UbcH5B and UbcH5C form thiol ester adducts with ubiquitin, and have activities similar to UbcH5A and AtUBC8 in the conjugation of ubiquitin to target proteins in the presence of the human ubiquitin protein ligase E6-AP. These results establish the existence of a highly conserved, and widely expressed, family of human ubiquitin conjugating enzymes.