Optimal molar ratios of antibody: ferritin: glutaraldehyde for the preparation of antibody-ferritin conjugates were investigated. The reaction volumes and the antibody concentration were held constant in twenty different reaction mixtures. The effect of five different antibody: glutaraldehyde ratios ranging from 1:25 to 1:400 and of four different antibody: ferritin ratios (1:1 to 1:01) on the yield of antibody-ferritin conjugates was tested in one-step reactions. The 20 different conjugates were tested by a newly developed gel precipitation technique, the inverse fused line rocket immunoelectrophoresis and by counting relative numbers of ferritin monomers and oligomers under the electron microscope. High concentrations of glutaraldehyde produced large heterogeneous precipitates as visualized in the gel technique. The relatively highest yield of ferritin-labeled antibodies without larger conjugates was produced by reacting antibodies, ferritin and glutaraldehyde at molar ratios of 1:1:100. The FRT-antibody conjugates produced at these molar ratios were partly abe to precipitate with their respective antigens.