Serological analysis showed that protection against the lethal H5N1 challenge correlated well with the observed antibody levels against sH5 3 as determined by ELISA (Fig. 3C-D) and by HI assay (Fig. 3E-F) . Both assays revealed a dose-dependent antibody response, which was substantially enhanced after the booster immunization. Relatively high HA antibody levels were observed after two immunizations, even with the lower dose, except in two animals, one of which did not survive the challenge (Fig. 3C) . Also after a single immunization, significant antibody levels were measured, except again in the low dose group. Here, 5 animals had hardly measurable ELISA titers. Consistently, 4 of these animals succumbed to the challenge. Also the one animal that died after a single immunization with 2 mg of sH5 3 did not have detectable sH5 3 antibodies. Essentially the same results as with the ELISA were obtained with the HI assay using sH5 3 as the hemagglutinating agent ( Fig. 3E-F) . Thus, the animals that succumbed to the lethal challenge after a single immunization also exhibited the lowest HI titers.