The in-vitro activity of the new parenteral cephalosporins, HR 810 (HR) and BMY-28142 (BMY), was compared with that of other beta-lactam antibiotics, including cefotaxime, against a total of 315 recent clinical isolates and characterized beta-lactamase producers. An agar dilution procedure was used to determine MICs and two inocula (10(4) and 10(6) cfu) were used throughout. Against all species of the Enterobacteriaceae tested, both HR and BMY were as active as, or slightly more active than, cefotaxime. HR differed from cefotaxime mainly in being eight-fold more active against Pseudomonas aeruginosa and two-fold more active against Staphylococcus aureus. BMY was also eight-fold more active than cefotaxime against Ps. aeruginosa but was two-fold less active than the latter against Staph. aureus. Both new compounds had good activity against Haemophilus influenzae, including the beta-lactamase producing strains of that species, and both had poor activity against the Bacteroides fragilis group (MIC90 greater than 128 mg/l).