The effects of minor illness on infants' social behavior were studied among disadvantaged infants who attended a day-care program. Twelve infants aged 4 1/2 to 6 months were observed once while playing alone and once while interacting with their teachers each week for 6 weeks. A time-sampling procedure was used to record frequencies of vocalizations, visual regard, touching, toy manipulation and infant's crying. Infant health status on the days of observation was determined by a pediatrician who reviewed medical records after the observational data were collected. Paired t-tests were used to compare behavior while infants were ill and well. In the playing alone situation, none of the differences between ill and well conditions for any of the measures were significant. In the teacher interaction situation, during illness infants vocalized significantly less and were responded to by teachers significantly less than when the infants were well. Results suggest that mild illness had no effect on infants' behavior. Since the effects in the illness condition appeared only in the teacher interaction situation the change in infant vocalization may have resulted from changes in teacher behavior rather than illness. It is suggested that teacher expectations may lead to unnecessary reductions in the level of social stimulation when children are ill. (Author/RH)