HR810 (Hoechst-Roussel Pharmaceuticals Inc., Somerville, N.J.) is a new, cyclical-pyridinium cephalosporin that appeared superior to numerous comparison drugs against 658 strains of aerobic and facultative anaerobic bacteria. Seventeen Enterobacteriaceae spp. were tested by broth microdilution methods, and the 50% MICs (MIC50s) and 90%o MICs (MIC90s) were 0.03 to 0.12 and 0.03 to 2.0 jxg/ml, respectively. Only one strain had an MIC >8.0 ,ug/ml (99.6% is considered susceptible). HR810 inhibited 98% of Pseudomonas aeruginosa isolates at <16 ,ug/ml, and the MIC90 for Acinetobacter spp. was 4.0 ,ug/ml. It was also very active against Pseudomonas spp. and Staphylococcus aureus (MIC90, 0.5 jLg/ml) but marginally active against methicillin-resistant staphylococcal strains (MICgo, 16 1Lg/ml) and enterococcus (MIC90, 32 jig/ml).Non-enterococcal streptococci had MIC50s ranging from 0.008 ,ug/ml for Streptococcus pyogenes to 0.12 tg/ml for pneumococci. All MICs of HR810 against I-Jaemophilus and Neisseria spp. were s0.03 jig/ml (MIC50, 0.002 to 0.008 ,ug/ml). HR810 poorly inhibited ,B-lactamases and was very stable against 11 tested P-lactamases of plasmid (TEM, OXA, SHV-i, and PSE) and chromosomal (Kl, K14, P99) types.HR81O (Hoechst-Roussel Pharmaceuticals Inc., Somerville, N.J.) is a new 3'-pyridinium-substituted cephalosporin (Fig. 1). It has a 2,3-cyclopentenopyridine instead of the 3'-acetoxy group found on cefotaxime sodium, and the compound is produced as a sulfate salt. Preliminary reports by the manufacturers, Hoechst AG (Frankfurt, Federal Republic of Germany) and Roussel-tJCLAF (Paris, France), indicate that the compound has marked antimicrobial activity against Pseudornonas aeruginosa, Staphylococcus aureus, enterococci, and members of the family Enterobacteriaceae;it is stable to P-lactamases and penetrates to penicillinbinding protein target sites, with binding comparable to that of ceftazidime; and it is bactericidal, is primarily excreted in the urine, and has a prolonged serum half-life in the early animal studies (2,4,14,22 574, 1983).In this in vitro evaluation, we compare the antimicrobial activity of HR810 with that of new ,B-lactams and also compare these results with those previously published for chemically similar drugs (1, 5-7, 10, 12, 20, 21). We also report the results of studies of P-lactamase stability, 1B-lactamase inhibition by HR810, and the effect of salt content in the medium on the HR810 MICs against methicillinresistant S. aureus strains.