The binding of 15 125I-labelled mouse monoclonal antibodies to cell-surface sialoglycoprotein alpha (SGP alpha: synonym Glycophorin A) was studied using intact IgG and Fab fragments. It was estimated that the number of sialoglycoprotein alpha (SGP alpha) molecules per red cell is of the order of 1 x 10(6) and the number of sialoglycoprotein delta (SGP delta; synonym Glycophorin B) molecules per red cell is of the order of 1.7-2.5 x 10(5). Competitive binding assays showed that antibodies of the same blood group specificity (four anti-Ns reacting with SGP alpha and SGP delta (BRIC 33, BRIC 115, BRIC 120, BRIC 123) and four anti-Wrbs reacting with SGP alpha (R7, BRIC 14, BRIC 89, BRIC 93) inhibited binding of each other to red cells. Two antibodies (R1.3 reacting with SGP alpha and SGP delta and R18 reacting with SGP alpha) recognized distinct epitopes, and the remaining five antibodies (BRIC 116, BRIC 117, BRIC 119, BRIC 127, R10 reacting with SGP alpha) partially inhibited binding of each other to red cells. This latter observation and the finding that four of these antibodies (BRIC 116, BRIC 117, BRIC 119, BRIC 127) bind to a considerably smaller number of antigen sites (1.69-2.71 x 10(5) for intact IgG) than the maximum value obtained, suggests heterogeneity of glycosylation within SGP alpha molecules. The functional affinities of the IgG antibodies ranged from 1 x 10(5) to 4 x 10(7)M-1.