A strain of Escherichia coli (MG/32) was recovered from the blood of a patient who had received ceftazidime for eight weeks. The isolate was resistant to ceftazidime but susceptible to other third-generation cephalosporins. Alterations in outer membrane proteins were implicated in this selective ceftazidime resistance.As ceftazidime susceptibility was regained, the quantity of outer membrane proteins of 37,000 and 39,000 molecular weight increased. Although the isolate possessed a TEM-1 β-lactamase, this enzyme was not involved in the selective resistance to ceftazidime; it did not disappear on reacquisition of ceftazidime susceptibility and did not hydrolyze the drug. Potassium c1avulanate enhanced the activity of ceftazidime against E. coli strain MG/32, but this enhancement was due to a direct effect on outer membrane proteins and not to β-Iactamase inhibition.