The first spanish outbreak of bacterial strains showing resistance to third generation cephalosporins and due to the presence of the extended spectrum beta-lactamase SHV-2 is reported. This outbreak was observed in Madrid during the years 1988-1990 and involved the San Carlos University Hospital with the same type of isolates at the Ramón y Cajal University Hospital.The screening for extended-spectrum beta-lactamases was performed by the double-disk synergy test. Analytical isoelectric focusing and susceptibility tests were performed in all the strains showing a presumptive extended-spectrum beta-lactamase.Fifty-nine strains belonging to four bacterial species (Klebsiella pneumoniae, 61%; Serratia marcescens, 31%; Klebsiella oxytoca, 5%, and Escherichia coli, 3%) showed a beta-lactamase of point isoelectric 7.6; the susceptibility tests demonstrated more resistance to cefotaxime and ceftriaxone than to ceftazidime and aztreonam.The biochemical, kinetic and isoelectrofocusing parameters demonstrated the presence of a SHV-2 enzyme. The blind application of NCCLS breakpoints would lead to false "susceptibility" results in over 40% of the cases.