A panel of thirteen monoclonal antibodies (Mabs) was assembled that reacts with surface antigens on eight of elevenLactobacillusbrewing spoilage organisms, including one or more ofL.brevis,L.buchneri, L.casei-alactosus,L.plantarum, or unspeciated isolate(s). Immunoblotting was done to identify the antigens involved in Mab binding. Antigen stability in situ was tested by protease treatment and by surface antigen extraction of washed bacteria. Protease susceptibility of extracted surface antigens was also examined. In most cases,Lactobacillussurface antigens detected by the Mabs appear to be noncovalently bound proteins readily altered or removed from the bacterium by various environmental conditions. This research identifies brewing conditions that need to be tested to ascertain whether bacterial surface antigen-reactive Mabs can be used for the rapid, sensitive, and specific detection ofLactobacillusbrewing spoilage organisms.Key words: brewing spoilage organism, immunoassay,Lactobacillus, monoclonal antibodies, surface antigens.