A rapid rate-kinetic turbidometric assay for the quantitation of viral complement fixing antibodies has been developed, using adenovirus as a model. The procedure is based on the turbidometric quantitation of intact sheep erythrocytes and measures the rate of hemolysis (change in absorbance at 640 nm/min/), at maximum velocity, occurring in the presence of residual complement not fixed by the antigen-antibody reaction. Reagents were standardized and assays performed using a microprocessor-controlled spectrophotometer with kinetic assay capability and a thermoregulated cell compartment. Eleven sera were assayed for complement fixing antibodies both by the conventional microtiter technique and by the rapid turbidometric method described here. Good correlation (r = 0.89) was obtained between the two procedures. Unlike the conventional complement fixation test, the rate-kinetic turbidometric complement fixation assay was found to be tolerant of variation in complement and antigen concentration, endpoint titers were objectively quantitated and, once reagents had been standardized, results could be obtained within 45-60 min. The technique is potentially adaptable to large-scale automation.