Monoclonal antibodies and thin-layer chromatography were used to study the unusual erythrocyte Lewis phenotypes found in healthy Polynesians. A single monoclonal anti-Leb reagent 073 (clone LM129) was found which could detect Leb antigen on the Polynesian erythrocytes of samples that were unreactive with various polyclonal and monoclonal anti-Leb reagents. Glycolipid fractions prepared from the plasma and erythrocytes of selected Polynesian samples of red-cell Le(a-b-), Le(a+b-) and Le(a+b+) phenotypes were found to have Leb glycolipids. The Leb antigen in some individuals is so weakly expressed that it is undetectable by routine erythrocyte phenotyping. Unusually large glycolipids bearing the Leb epitope were also found in some Polynesian samples, although the contribution of these novel glycolipids to phenotyping is unclear. The inability to detect Leb by routine methods and the presence of novel structures can be partially explained in terms of the presence of a weak secretor gene Se(w).