A field study using five different private periodontal practices was conducted; it compared two microbiologic culture samples simultaneously secured from the same sites within 23 individual patients and submitted for bacterial identification and antibiotic sensitivity testing to two separate laboratories. The results from the two laboratories were often different. In no instance did both laboratories agree on the presence of identical bacterial species. When only bacteria above threshold levels were compared, agreement was found in only nine of 23 cases. When examining antibiotic sensitivity, using 100% kill of all tested pathogens as the ideal, agreement between the two laboratories was poor. The laboratories agreed on the use of amoxicillin 17% of the time, tetracycline 26% of the time, and metronidazole 48% of the time. The use of amoxicillin and metronidazole in combination yielded a 78% agreement when the results of both laboratories were combined. It would appear from the data that the empirical use of amoxicillin-metronidazole combination therapy may be more clinically sound and cost effective than culturing and antibiotic selection based on the results of culture from any single microbiologic testing laboratory.