Results of coagulation studies on 21 homozygote patients with factor XII (FXII) deficiency revealed that all of them had no cross-reacting material (CRM) in their plasma. The 58 heterozygotes had in every instance an antigen level comparable to that of clotting activity namely, approximately 50% of normal. An analysis of all pertinent literature also showed that the presence of CRM is very rare in FXII deficiency. CRM is present in approximately 5% of homozygote patients. More precisely, seven of 145 patients. Only in one case, the antigen level was normal (FXII Washington). This prevalence appears lower than that observed for another contact phase factor (prekallikrein). The significance of blood abnormal forms of FXII has not been completely clarified yet. Their study appears useful in the attempt of clarifying the structure-function relation of factor XII.