The Rh (Rhesus) protein family is expressed mainly in cells of erythroid lineage and, currently, is thought to comprise Rh30 (RhD and RhCE) polypeptides and Rh50 glycoprotein. As multipass integral membrane proteins, Rh30 and Rh50 are closely related by clear sequence homology and similar membrane topology, functioning respectively as the carrier and coexpressor of the Rh blood group antigens. The past year has seen a further accumulation of evidence concerning the molecular basis of Rh antigen expression, a major expansion of the catalogue of Rh allelic polymorphism, and significant progress in defining the genetic defects underlying the Rh deficiency syndrome. Now the gene structure for many Rh variants has been determined and some information obtained on the mechanisms of Rh genetic diversity and on the factors affecting the formation and surface presentation of discrete antigenic epitopes. The identification of various mutations in the Rh50 gene associated with the Rhnull and Rhmod phenotypes establishes RH50 as a suppressor for the RH locus and Rh50 protein as a critical component of the Rh complex. These new advances have broadened our understanding of the molecular biology of the Rh protein family and provided insight into the functional role of the Rh complex in maintaining the architecture of the erythrocyte membrane.