BACKGROUND Strains of Escherichia coli are frequently plasmid carriers. In this species, resistance to beta-lactam antibiotics is almost always conditioned by the production of enzymes coded by plasmidic genes. The present is a study of the plasmids of 44 ampicillin-sensitive strains and 134 ampicillin-resistant (ampS and ampR). The possibility that the number and size of the plasmids are different and that this data may be added to the information to be considered in these two groups of strains is suggested. METHODS The 178 strains selected had been isolated from human products. Sensitivity to ampicillin was studied by diffusion and was confirmed with the study of MIC (Mueller-Hinton agar, innoculum: 5 x 10 CFU). The plasmid type beta-lactamases were identified by analytical isoelectrofocus. Characterization of the plasmids was performed according to a variant of the Birnboim and Doly alkaline lysis technique. RESULTS Among the ampR and ampS strains no plasmid were observed in 9 (6.72%) and 11 (25%) respectively. The mean number of plasmids was 2.53 and 1.57, ranging between 0-10 and 0-5. The number of strains with plasmids larger than, or equal to, 38 Kb was 113 and 27 respectively. The largest plasmids observed in the ampS strains were of 99 Kb and in the ampR of 109 Kb. A total of 3.73% of the ampR strains presented plasmids larger than 99 Kb and 8.20% more than 5 plasmids. CONCLUSIONS No plasmids, presence of up to five and sizes smaller than or equal to 99 Kb were observed in strains of ampS and ampR. The presence of more than five and/or plasmids larger than or equal to 100 Kb was observed in 11.94% of the ampR.