Sensitivity tests of Mycoplasma pneumoniae to antibiotics were performed with 28 strains to determined the minimum inhibitory concentration (MIC) of the antibiotics, the MIC after 14 days' incubation at 37 C, and the minimum plasmacidal concentration. The antibiotics tested were erythromycin, tetracycline, oxytetracycline, chloramphenicol and streptomycin. The MIC of erythromycin was around 0.01 microgram/ml, whereas that of tetracycline was approximately 0.1 microgram/ml. Of the 28 strains tested, nine isolates gave rise to mutants resistant to high concentrations of erythromycin, chloramphenicol and streptomycin; none of the strains produced mutants resistant to tetracycline. Of the nine resistant mutants, three strains were isolated from erythromycin-treated patients. One strain, isolated from a patient treated with erythromycin for 18 days, was resistant to the antibiotic in a concentration greater than 100 microgram/ml.