In 2784 specimens submitted to the Clinical Microbiology Laboratory of the Instituto Nacional de Neurología from May 1989 to January 1990, 140 (5.0%) had gram negative bacilli (GNB) resistant to five or more antibiotics. One hundred different isolates recovered from urine, tracheal aspirates, blood, cerebrospinal fluid, surgical wound infections and other sites were studied. Pseudomonas spp, Enterobacter spp, Klebsiella pneumoniae and Escherichia coli accounted for 86% of the total. Disk diffusion susceptibility tests with the NCCLS method revealed 59 to 67% of isolates susceptible to amikacin, ciprofloxacin and ceftazidime. Sixteen to 30% were inhibited by piperacillin, cefoperazone and cefotaxime. Less than 10% were susceptible to carbenicillin, tobramycin, gentamicin, cephalothin and ampicillin. Minimum inhibitory concentrations (MIC) to amikacin and ciprofloxacin were determined by broth macrotube dilution in Mueller-Hinton broth supplemented with Ca and Mg ions with a 5 x 10(5) cfu/mL inocula. MIC of 16 ug/mL or less for amikacin was found in 39% of the isolates; MIC 50 was 32 ug/mL and MIC 90 greater than 64 ug/mL. Ciprofloxacin had MIC 50 of 1 ug/mL and an MIC 90 greater than 4 ug/mL. Higher MICs to ciprofloxacin were seen in Pseudomonas spp.