Longitudinal observations were made of a well-defined population of children at a day care center in an investigation of the pathogenesis of infections due to respiratory syncytial virus (RSV) and Mycoplasma pneumoniae. A single RSV infection induced a modest but significant degree of resistance to further RSV infection in these children. Age and immunity seemed to interact to decrease the intensity of the clinical expression of illness associated with RSV infection. Infants and young children had asymptomatic or mild infections with M. pneumoniae; some of these children also became reinfected. A rise in titer of antibody to M. pneumoniae was demonstrated frequently in children of all ages. However, stimulation of peripheral lymphocytes by M. pneumoniae antigen was demonstrated infrequently in children younger than four years of age but frequently in children older than four years of age. It is speculated that the clinical expression of disease due to M. pneumoniae is modulated by immune responses; this hypothesis would explain the greater severity of illness in older children and young adults than in younger children. It is also speculated that RSV vaccines will not prevent RSV infection but may be expected to lessen the severity of clinical disease that follows such infections. M. pneumoniae vaccines probably should not be used in children because these vaccines may enhance immunity and increase the sevrity of illness.