Membranes from erythrocytes with a new Gerbich (Ge)-negative phenotype (Leach phenotype), as well as those from two other Ge-negative phenotypes, were examined. Whereas cells of the Leach phenotype apparently lack three minor sialoglycoproteins (beta, beta 1 and gamma), the membranes of Ge- Yus- and Ge- Yus+ erythrocytes apparently lack beta- and gamma-sialoglycoproteins but contain additional diffusely migrating components of apparent Mr 30 500-34 500 and 32 500-36 500 respectively. Immunoprecipitation experiments showed that the abnormal components of both Ge- Yus- and Ge- Yus+ erythrocytes reacted with two monoclonal antibodies, BRIC 4 and BRIC 10. These antibodies have been shown to react with sialoglycoproteins beta and beta 1 in normal erythrocytes. Cytoskeletal preparations from Ge- Yus- and Ge- Yus+ erythrocyte membranes contained the abnormal components. In contrast with cells of the Leach phenotype, which are elliptocytic, Ge- Yus- and Ge- Yus+ were of normal shape, despite their apparent lack of beta- and gamma-sialoglycoproteins. It seems likely that the abnormal components in these cells contribute to their normal shape. Ovalocytic erythrocytes were shown to incorporate more radioactivity in the sialoglycoprotein-beta 1 region than normal erythrocytes after labelling by the periodate/NaB3H4 technique. It is suggested that abnormal components in Ge- Yus- and Ge- Yus+ erythrocytes result from chromosomal misalignment with unequal crossing-over at meiosis between the genes giving rise to beta-, beta 1- and gamma-sialoglycoproteins.