A serologic survey for antibodies to Borrelia burgdorferi was conducted on sheep and goat serum samples collected in Alto Adige-South Tyrol, Italy, in 1990. Sera were tested by Indirect Immune Fluorescence Assay (IIFA) and Microbial Adherence Immobilization Assay (MAIA). IIFA and/or MAIA anti-B. burgdorferi antibodies were detected in 14.1% of the 269 sheep and 36.8% of the 133 goats examined. IIFA and MAIA were both positive in 4 out of 38 positive sheep sera (10.5%) and 21 out of 49 positive goat sera (42.8%). These discrepancies suggest that MAIA- and IIFA-detected antibodies do differ from each other. The detection by MAIA of antibodies sensitizing B. burgdorferi to the killing effect of complement seems to be a valid parameter to evaluate the acquired immunity of sheep and goats to B. burgdorferi infections.