A 21-year-old college student complained of malaise, low-grade fever, and a harsh cough, but not of muscle aches and pains. An x-ray revealed a diffuse interstitial pneumonia in the left lobes of the lung. The WBC count was normal. The student has been ill for a week. The following laboratory data were available within 2 days: cold agglutinins--negative; complement fixation (M. pneumoniae)--1:64; viral culture--pending, but negative to date; bacterial culture of sputum on blood agar and MacConkey's agar--normal oral flora. In order to confirm the diagnosis, which of the following procedures could be ordered to achieve a specific and sensitive diagnosis?