The change in antimicrobial resistance of organisms isolated from blood cultures was studied in a university hospital that opened in July 1977 and had 550 beds by December 1982. Minimal inhibitory concentrations (MIC) were studied by an automatic microdilution technique. Analysis of MICs in the different groups of organisms in relation with time demonstrated no enhancement of resistance during these first 5 1/2 years, except for Pseudomonas aeruginosa that became significantly more resistant (p less than 0.05) to five of the seven antibiotics tested. However, comparison of the two periods July 1977-December 1980 and january 1981-December 1982 evidenced a trend towards a decrease in Enterobacteriaceae (p. less than 0.001) and increase in P. aeruginosa (p less than 0.01) and S. aureus (p less than 0.001). This suggests that the selective pressure from hospital use of antibiotics caused no increase in resistance of different organisms but rather a modification of their relative frequency.