SummaryConcentrations of C3b inactivator (C3bINA) and β1H globulin, two proteins which regluate turnover of the alternative complement pathway, have been measured in the sera of 38 patients with Gram‐negative bacteraemia at the onset of fever, prior to the onset of shock. The mean serum concentrations of these two proteins was reduced compared with normal controls (t = 1.8250, P<0.05 for C3bINA; t =3.9555, P<0.005 for β1H These reductions were most pronounced in those patients who subsequently developed shock, with or without a fatal outcome. Serum levels of C3bINA and β1H correlated well with levels of C3 factor B, but only poorly with C4. C3bINA, but not β1H, also correlated with properdin. As a result of these findings it has been suggested that low serum concentrations of C3bINA and β1H in Gram‐negative bacteraemia, may predispose to excessive alternative pathway