A selective deficiency of properdin (P) was identified in a 58-year-old male and in his 29-year-old nephew, both of whom were clinically healthy. As determined by different immunochemical methods P at low concentrations (about 2 mg/1) was detectable in serum and plasma. Three female relatives, including the mother and daughter of one of the P- deficient males showed moderately low P concentrations. The findings clearly suggested that the deficiency was inherited as an X-linked trait. Three males belonging to another family with P deficiency also showed detectable P concentrations. By contrast, no P (<0.1 mg/1) was found in 8 males belonging to three other families. We suggest that there are two variants of X-linked P deficiency; P deficiency type 1, characterized by extremely low P concentrations (< 0.1 mg/1); and P deficiency type 2 recognizable by P concentrations of about 2 mg/1. The P detected in P deficiency type 2 had subunits of normal molecular weight (52 kilodal- tons), but eluted in a lower molecular weight range than did the P of normal serum, either on gel filtration (Ultrogel AcA 22) or on size exclusion chromatography (TSK-4000). The evidence suggested that the P concentration may be one determinant of P oligomer formation. P-deficient serum type 2 did not support fluid phase C3 cleavage in the presence of such alternative pathway activators as inulin and zymosan, nor did it support efficient lysis of guinea pig erythrocytes in agarose gel. By contrast, rabbit erythrocytes were efficiently lyzed, but at a slow rate. P-deficient serum type 1 did not support lysis of rabbit erythrocytes in the assay system used. The reaction was clearly promoted by very low concentrations of purified P. Partially purified P from a male with P deficiency type 2 was shown to be hemolytically active. Further evidence of P function in P deficiency type 2 was obtained by using IgG- presensitized serogroup W-135 meningococci in an alternative pathway-mediated serum bactericidal assay.