Purified factor B heated at 50°C for 30 min (ΔB) at physiological concentration (200–400 µg/ml) or higher sedimented as at least 4–5 species in linear sucrose density gradients. The lightest species of ΔB possessed slightly lower sedimentation rate, 5.3S, than native B, 5.8S. Consecutive heavier species differed by ∼ 2.6S. Serum fortified with radiolabeled 131I-B was heated; 131I-ΔB distributed broadely in the gradient. In analytical polyacrylamide disc gel electrophoresis (PAGE), ΔB migrated as multiple bands with the same or slower mobility compared with B, indicative of a polymerization phenomenon. Prior to PAGE, samples of ΔB were treated overnight at room temperature with 1% sodium dodecyl sulphate (SDS) with or without 8 M urea (Ur). This reduced the intensity of the slow bands and, concomitantly, increased the quantity of the fastest band. Upon additional treatment with 1% beta-mercaptoethanol (2-ME), ΔB migrated as a single band with the same mobility as B.