SummaryMycoplasma flora of the pharynx and anterior oral pool of 16 patients with early infectious mononucleosis did not differ significantly from that of 12 controls. Patient-derived strains of M. pharyngis, M. salivarium, M. hominis, and T-strain mycoplasma were employed in metabolic inhibition antibody tests of paired and single sera from patients and controls. The only difference observed was the presence of MI antibody titers of ≥1:4 to M. pharyngis in 40% of 42 patientsapos; sera compared to 14% to 22 sera from controls. Antibody responses to mycoplasmas were not observed during the course of illness.