Bioluminescence is a subcategory of chemiluminescence in which the energy-generating chemical reaction is enzyme catalyzed; enzymes are produced only by living systems. For the clinically important Gram positive cocci, however, adenosine triphosphate estimation of cell numbers by bioluminescence may not give good agreement with the colony count on an agar plate. A number of urine screening techniques have been used. In the bioluminescence antibiotic susceptibility testing (AST), bacteria are grown in broth in the presence or absence of antibiotics. Preliminary experiments using the bioluminescence technique were reported at the Annual Meeting of the American Society for Microbiology in 1985. The Los Alamos Diagnostics approach has been to try to develop a bioluminescence AST test that is practical for routine use in a clinical microbiology laboratory. Results of the bioluminescence AST were compared to the results obtained by the disk diffusion standard method in both Los Alamos and Tucson, and to the Abbott Avantage in Tucson.