Two strains of Bartonella bacilliformis were cultured on Columia agar supplemented with 5% defibrinated human blood. Antigens prepared from these cultures were used for determination of antibodies by the fluorescence antibody test (FAT), indirect haemagglutination (IHA) and an enzyme immunoassay (ELISA). One hundred and eighty-seven human sera from B. bacilliformis-endemic areas of Peru were tested of which 63.6% were reactive. ELISA was the most sensitive test, followed by FAT and IHA. IgM antibody was determined by FAT with the IgM fraction of test sera. It was present not only in patients with Oroya fever but also in some healthy individuals as well as in one patient with chronic bartonellosis (verruga peruana). Substantial differences in antigenic activity between the two strains of B. bacilliformis were not observed. Cross reactivity with sera containing antibodies to bacteria other than B. bacilliformis was not noted. For identification of Oroya fever patients in the field, an eosin/thiazine stain of blood smears was found to be appropriate.