Subjectively healthy persons were investigated for the presence of Mycoplasma pneumoniae in throat cultures. During a peak period of M. pneumoniae incidence, 13.5% of 758 healthy volunteers were found to harbour the bacterium in the throat. The investigation was continued, and during a subsequent period of 11 months, the incidence of M. pneumoniae isolated decreased to 4.6% of 499 volunteers. All new blood donor sera 1990-1991 (422 sera) were screened for the presence of antibodies to M. pneumoniae; it was found that there was a fluctuating but significant number of individuals with positive serology based on a single test occasion.