Two cases of mycoplasma pneumonias are presented. Initially this etiology was not suspected. Prior to hospitalization they were treated with cefamandole and carbenicillin and cephradine with gentamycin without any improvement in the clinical state of the patients. After serological tests finding a high titer of serum antibodies (1: 2560) the therapy was altered in the first patient to minocycline (in a dose of 200 mg per day for 16 days) and in the second to doxycycline (in a dose of 200 mg per day for 10 days). The first patient improved clinically after two days of altered therapy, the second after 10 days. Both patients were discharged fully recovered after 32 and 36 days of hospitalization.