Metabolic studies were performed with a purified, functionally-active preparation of human beta 1H. In seven normal human subjects, the half-life ranged from 66--87 hr with fractional catabolic rates (FCR) of 1.04--1.63%/hr. Synthesis rates were 0.22--0.57 mg/kg/hr and extravascular distribution ratios were 0.34--0.67. There was evidence of extra-vascular catabolism in each subject. In sixteen patients with immunological disease four showed hypercatabolism of beta 1H. However, three patients with C3 mephritis factor (NeF) had normal beta 1H turnover despite profound reduction in C3 concentration; it is suggested that the reaction of beta 1H with the C3b. Bb convertase exposes it to a catabolic site and that in the NeF patients the NeF stabilized convertase prevents such exposure. Studies of the acute phase response were carried out in nine patients following elective surgery, with C-reactive protein (CRP) used as the control protein: six patients showed no rise in beta 1H levels and three showed a small (20%) rise whereas all exhibited a gross rise in CRP. Pre-incubation of 125I- beta 1H with NHS, with NHS in the presence of NeF and with C3b+C3b 1NA caused no change in beta 1H turnover in animals despite demonstrable total C3 conversion with the NeF.