The beta-lactamase stability and inhibitory activity of 1-oxa cephalosporin, (6R,7R)-7-[[carboxy(4-hydroxyphenyl)acetyl]amino]-7-methoxy-3-[[(1-methyl-1H-tetrazol-5-yl)thio]methyl]-8-oxo-5-oxa-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid, was investigated and compared to that of cefoxitin and cefotaxime. There was no detectable beta-lactamase hydrolysis of 1-oxa cephalosporin, cefotaxime and cefoxitin when incubated with beta-lactamases of plasmid or chromosomal origin which were primarily cephalosporinases or enzymes which hydrolyzed both penicillins and cephalosporins. The beta-lactamase inhibitory activity of 1-oxa cephalosporin was comparable to that of cefoxitin and cefotaxime. At equal molar concentration of substrate and inhibitor, cefoxitin, cefotaxime and 1-oxa cephalosporin effectively inhibited cephalosporinase hydrolysis of cephaloridine. Cefoxitin and cefotaxime were more effective inhibitors than the 1-oxa cephalosporin against a Providencia enzyme, whereas cefotaxime and 1-oxa cephalosporin were more effective inhibitors of a Citrobacter cephalosporinase.