Transmission of infection with rhinovirus type 55 was attempted under natural circumstances of interaction among 26 experimentally infected donors and 33 antibody-free (titer, less than 1:3) recipients. In a total of three experiments, only two recipients (6%) became infected. In the first experiment no transmissions from five donors to nine recipients occurred after 2-3 hr of loud vocalization and card playing in a small room. In the second experiment a cold was transmitted to one (9%) of 11 recipients living in dormitory rooms for 36 hr in groups consisting chiefly of two donors and two recipients. In the third experiment one (8%) of 13 recipients was infected after kissing an infected donor. In studies with rhinovirus type 16, the 50% human infectious dose was found to be 0.28 TCID50 (50% tissue culture infectious dose) in the nose, 2,260 TCID50 on the tongue, and 11,000 TCID50 on the external nares. Rhinoviral infections are difficult to transmit by short-term natural exposure, perhaps because the agent must be present in overwhelming numbers to reach susceptible mucosal cells.