The Iodogen technique was compared with the microelectrochemical technique for the radioiodination of submilligram quantities of monoclonal antibody 140.240. The titre of the antibody against cultured melanoma cells was decreased five-fold after iodination with 0.80-0.85 atoms of iodine per molecule of antibody with both techniques. The Iodogen technique resulted in 82.1% incorporation of iodine into the antibody, somewhat higher than the 78.0% obtained with the microchemical technique (P = 0.10). Therefore, the Iodogen technique is the preferred method based on efficiency and convenience.