Limited data are available about the pharmacokinetics of placenta-derived factor XIII (FXIII) concentrate in patients with FXIII deficiency. This concentrate contains only the active subunit A but not the carrier subunit B of the factor, and perplexities have been raised about its clinical use. Moreover, no data are available on its use in the rare patients completely lacking both subunit A and subunit B. Therefore, we evaluated the pharmacokinetics of a commercial placenta concentrate in three patients with FXIII deficiency: two lacking subunit A (type II) and one lacking both subunits (type I). The elimination half-life of the infused placenta subunit A in the three patients was very similar (280, 283, and 272 hr) and was also consistent with the previously reported data for plasma-derived FXIII. No thrombin-independent activity was observed in our concentrate batches. The recovery was significantly lower in the type I patient, in whom infusion of subunit A was not able to elicit a monthly increment of subunit B, as usually observed in type II patients. Monthly infusions of placenta concentrate (at higher dosage in type I patient) have been administered to our patients for two to three years and no evidence of inhibitor against factor XIII activity has been observed. We conclude that placenta concentrates may be as effective as plasma derivatives in replacement therapy of factor XIII deficiency, even in patients who lack subunit B.