A total of 90 isolates of Haemophilus were studied for susceptibility to tetracycline, doxycycline and minocycline. In vitro activity was determined by agar dilution method (MIC) and disk diffusion test. Disk diffusion tests were done on chocolate agar and on supplemented Mueller-Hinton agar. MICs rang from greater than or equal to 128 to 0,25, 32 to 0,5 and 32 to 0,25 micrograms/ml for tetracycline, doxycycline and minocycline respectively. Strains can be divided into tetracycline-sensitive strains which are inhibited by concentration less than or equal to 2 micrograms/ml and tetracycline-resistant strains inhibited by greater than or equal to 16 micrograms/ml. Sensitive strains are inhibited by 0,5-1 microgram/ml of doxycycline and 0,25-2 micrograms/ml of minocycline. Resistant strains are inhibited by concentrations from 2 to 32 micrograms/ml of doxycycline and from 0,5 to 32 micrograms/ml of minocycline. All but one resistant strains were inhibited by a concentration of less than or equal to 4 micrograms/ml of minocycline; all but two resistants strains were inhibited by doxycycline at a concentration greater than or equal to 4 micrograms/ml. The correlation coefficients are the following: for tetracycline, r = -0,95; for doxycycline r = -0,83 when disk diffusion test is done on supplemented Mueller-Hinton and r = -0,94 on chocolate agar; for minocycline r = -0,62 on supplemented Muller-Hinton and r = -0,75 on chocolate agar.