We describe here a simple, reliable and quantitative method to measure the alternative pathway (AP) dependent mode of complement activation for the lysis of rabbit erythrocytes. In the test the reciprocal of the plasma volume needed to destroy 50% of available rabbit erythrocytes is defined as the functional measure of this activity (AP50). The test was found to be highly reproducible both within and between assays with a coefficient of variation which was less than 5%. Sensitivity was also shown to be satisfactory and all of the plasma samples from the healthy blood donors which were tested could be assayed with precision. The specificity of the AP50 assay for AP complement activation was verified by the fact that a C4 affinity-depleted plasma gave an AP50 value within the normal range (52.8 U/ml) while a similar aliquot of the same plasma, affinity-depleted of factor B, gave an undetectable AP50 value (less than 10 U/ml). Furthermore, a sample was unable to lyse the target cells when heated to 50 degrees C or 56 degrees C, treatments which are known to destroy factor B and total haemolytic complement, respectively. To ensure inhibition of the classical pathway of activation. EGTA and MgCl2 were added during the assay. An advantage of this assay is that it is possible, using microplates, multipipettes and a spectrophotometer coupled to a computer, to semi-automate the procedure.