The aims of this study were to identify the rate-limiting components and reaction steps in the integrated activation sequence of the alternative (AP) and classical (CP) pathways of the complement (C) system. In an initial correlation analysis we found that the haemolysis rate in AP was correlated with the concentrations of C5 and IgM. In CP, the haemolysis rate was correlated with the concentrations of C2-C6, factors I and B, and IgM. In order to identify the rate-limiting components, we added single, purified C components and IgM to pooled, normal human serum and measured the resultant change in the haemolysis rate. We found that a large number of different components, rather than a single one, were rate-limiting in AP and CP. In reconstitution experiments we found that in CP the rate-limiting reaction steps are the activation of C4 and C2. In AP we cannot identify the rate-limiting step precisely, but can only state that it is at the C3 activation step or earlier.