This study was conducted to observe the effects of social class on the interaction of mothers and their 12-week-old infants. Data on the infants' cognitive and attentive behavior was also obtained. Each of 32 white and black infants from five different levels of social class was observed at home for two full hours of waking time. Observed infant behavior included move, vocalize, fret/cry, play, noise, and smile. Findings indicate that lower SES infants vocalize and smile more and fret/cry less than upper middle SES infants. Maternal behaviors of touch, hold, smile, look, and play were more frequent among lower SES than middle SES mothers, and lower SES mothers spend more time watching TV than the middle SES mothers. There is a relatively strong relationship between infant and maternal behavior. Middle SES mothers vocalize when their infants vocalize, touch and hold them when they fret and watch them play. Lower SES mothers tend to touch their infants when they vocalize, when they cry and when they are at play. There were no class differences on the two infant mental tests. Performance on a measure of attention indicated that two-thirds of the middle class infants failed to show response decrement while all the lower class infants demonstrated response decrement. In general, this study supports the presence of social class differences in terms of both cognitive and attentive behaviors.