The response to immunization against Leishmania braziliensis in rabbits was followed by measurement of antibody by enzyme-linked immunosorbent assay (ELISA), passive hemagglutination, complement fixation, and countercurrent immunoelectrophoresis. Passive hemagglutination and complement-fixation titers were interpreted according to standard methods, whereas ELISA exact titers were derived by the equation Y = a-b log X, which describes the straight line that results when absorbances are plotted against test serum dilutions. Peak titer was measured at 32 +/- 1 days after initial injection, irrespective of the assay method. After seven days, the reciprocal antibody titer was 1,130 by ELISA, 0 by passive hemagglutination, and 8 by complement fixation. Precipitin bands were shown by countercurrent immunoelectrophoresis only at peak titer. With sera from hamsters experimentally infected with Leishmania, 94% showed leishmanial antibody by ELISA, compared with 92% by complement fixation and 65% by passive hemagglutination. When 31 samples of sera from human patients with cutaneous leishmaniasis were tested only by ELISA, 23 were positive for leishmanial antibodies.