Serological observations have suggested that numerous D, many e (especially in Blacks), several E, and rare c variants exist within the Rh blood group system. The molecular basis for expression of many of these variants has been elucidated. This study describes five unrelated Caucasians whose red blood cells reacted with polyclonal anti-e but did not react with some monoclonal anti-e, which suggested that they carried a variant e antigen. Molecular investigation revealed the presence of a 48G-->C change (encoding cysteine instead of tryptophan at amino acid 16) in their RHce gene. No other differences were found, which suggests that amino acid residues located in the first transmembrane region can affect expression of the e antigen, whose critical residues are on the predicted fourth external loop of the protein. This polymorphism has not previously been observed because polyclonal anti-e does not distinguish this variant from wild type. This position is polymorphic in RHce alleles and the presence of the 48C nucleotide is often found in the R0 (Dce) haplotype.