Analysis of human serum for total hemolytic complement (CH50) is useful in following the course of patients who have diseases such as lupus nephritis or hypocomplementemic glomerulonephritis. A semi-automated system has been devised that facilitates test performance and reduces cost. An Autoprep dilutor is driven by an Apple IIe microprocessor. Samples are picked up, diluted with buffer, and added to a 96-well flat-bottom Linbro plate in a final volume of 100 microliters. Sensitized sheep RBC (100 microliters) are then added to each well. The plate is incubated at 37 degrees C for 90 min, agitated on a shaker, and then read by a Dynatech MR600 plate reader at 630 nm. The microprocessor converts the reading to percentage hemolysis by utilizing both a buffer blank and a complete hemolysis blank (saponin); these values are applied to the von Krogh equation to yield a CH50 value. Values obtained for normal human serum samples correlate well with those from an established semi-automated method (Technicon) in use in our laboratory.