The levels of protein C (PC) and antithrombin III (AT III) antigens (ag) were measured in the plasma of 39 patients with various histologic types of primary nephrotic syndrome (NS) and in 12 patients with amyloidosis secondary to familial Mediterranean fever (FMF). The controls comprised 15 healthy children. Normal or elevated PC levels were observed in primary NS patients (mean 64%, range 36-98%) and in amyloidosis patients (mean 58%, range 48-70%). There was no difference found between PC ag levels in primary NS and in amyloidosis patients. In addition, no correlation existed between protein selectivity and the PC ag levels in the primary NS patients. Normal and decreased levels of AT III were observed (mean 29 mg/dl, range 11.1-39 mg/dl) in the patients with primary NS and amyloidosis (mean 31 mg/dl range, 21-39 mg/dl). The AT III ag levels of these two groups did not differ and no correlation was found between protein selectivity and AT III levels in primary NS patients. These results suggest that in patients with primary NS, or amyloidosis secondary to FMF, hypercoagulability is not related to a deficiency in PC ag levels due to a dynamic balance between urinary losses, increased rate of hepatic synthesis, catabolism and the distribution of PC in the body compartments. Patients with low AT III levels may be more susceptible to thromboembolic complications than patients with normal levels.