Haemolytic assays in agarose plates for components of the classical complement pathway: interference by the alternative pathway.

It has been observed that when serum C6 is measured by the haemolytic radial diffusion technique a heat labile factor limits the size of the haemolytic rings. This reduction is haemolysis has been shown to be due to alternative pathway activation of C6 in the agarose plate; and that the heat labile factor is Factor B of the alternative pathway. This phenomenon is of practical importance when assaying for C6; however, it does not explain the observations of a C6 inactivator reported by Nelson & Biro (1968).