Despite the clinical-epidemiological features of plague, numerous suspected cases in Brazilian outbreaks have been discarded because of negative results from the hemagglutination test for antibodies against the Yersinia pestis F1 antigen. The transcendence of plague justifies studying whether such results are due to unresponsiveness to F1, and whether other Y. pestis proteins might be recognized in suspect serum. These would therefore be candidates to be alternative diagnostic targets to the F1 antigen. Thus, strains of Y. pestis and Y. pseudotuberculosis, a recombinant YopH protein and the F1 antigen were used to analyze serum from patients and immune serum from rabbits. F1 and YopH were not recognized by HA-negative human serum and no major protein common to all the human and rabbit serum samples was identified. This allows the conclusion that suspected cases must be subjected to more rigorous clinical-laboratory evaluation, with strengthening of epidemiological investigations in the search of other etiologies.