A non-competitive method for the determination of the C4 component of human complement in serum is described. This procedure involves use of a specific antibody covalently attached to derivatized polyacrylamide beads and a fluorescently labeled specific antibody. Reproducible results were achieved for C4 in serum in the range of 10 mg/dl to 170 mg/dl within 2 h. C4 levels as low as 150 ng/ml can also be measured. Fluorescent immunoassay and radial immunodiffusion were used to determine C4 levels in healthy adults. Good agreement was found between the two methods.