This paper deals with 254 monosera of patients with leptospirosis verified by macroagglutination test with thermoresistant antigen (TR antigen) and passive hemaglutination (HA), as reference technique. The TR antigen was elaborated according to the method described by Mailloux and coworkers, 1974, with modifications, and it showed a wide generic reactivity. It was specific when serologic test were performed with hyperimmune sera obtained from febrile disease producing bacteria and the causal agent of syphilis. The results obtained by both serologic tests were compared and it was observed that 106 sera (41.73%) were reactive to HA, which showed a highly significant statistical difference between the two techniques; so a lower sensibility of TR macroagglutination test was determined.