In 1954, cycloserine (Oxamycin, D-4--amino-3-isoxazolidinone) was discovered in three laboratories in the United States to be a fermentation product of three species of Streptomyces.1-3 A flurry of reports followed, indicating cycloserine was of good effect in the treatment of a wide variety of non-acid-fast4-14 as well as acid-fast15-18 microbial infections. However, there has been little recent interest in the application of cycloserine to the treatment of infections caused by non-acid-fast microorganisms in this country—in contrast with interest abroad.8,9,12-14 Lack of establishment of cycloserine as an antibiotic useful in treating other than mycobacterial infections relates to at least two factors: (1) by conventional in vitro susceptibility testing, non-acid-fast bacteria generally appear to be resistant to cycloserine; (2) as employed in tuberculotherapy, untoward reactions occur rather frequently. By simultaneous testing of 293 clinical isolates of bacteria representative of 11 bacterial groups, using two synthetic media which differed only