An antigen-antibody complex obtained by simple mixing of specific rabbit antiserum and Escherichia coli β-galactosidase is used as a “probe” to determine immune complexes in biological fluids in a novel competitive assay. The complex-recognizing unit, immobilized on plastic beads, is bovine conglutinin, a large protein with affinity for C3bi bound to the complex. The intrinsic enzyme activity of the probe provides an ecologically acceptable signal with high resolution. The new assay has been tested in clinical trials and, by using a battery of recognition units, it should provide a tool for “finger-printing” complexes found in different diseases according to their structures and functional characteristics.