The relationship of clinical bleeding tendency and factor XI antigen (XI:Ag) in factor XI deficiency was studied in 78 members of 25 factor XI-deficient kindreds. Factor XI:Ag was measured in a competitive radioimmunoassay, using monospecific, heterologous anti-factor XI antibody. 125I-labeled factor XI, and staphylococcal protein A as the precipitating agent. Deficiency of factor XI clotting activity (XI:C), less than 0.62 U/mL, occurred in 48 individuals, 22 of whom experienced postoperative or posttraumatic bleeding: Their mean factor XI:C was 0.21 +/- 0.04 U/mL (SEM), and factor XI:Ag was 0.23 +/- 0.04 U/mL. The remaining 26 had no clinical bleeding, many despite surgical challenge: Their mean factor XI:C was 0.30 +/- 0.04 U/mL, and factor XI:Ag was 0.34 +/- 0.05 U/mL. In all, 13 kindreds had between 1 and 11 members with bleeding; the other 12 had none with deficient hemostasis. Two heterozygous factor XI-deficient individuals appeared to be positive for cross-reacting material (CRM+). The slope of the regression line for factor XI:C and factor XI:Ag data points in the 78 individuals tested did not differ from control, and all points fell within 95% confidence limits derived from control. In conclusion, bleeding tendency appears to be consistent within a given kindred and is not determined exclusively by factor XI:C or factor XI:Ag levels.