This study examines the relationship between mother-infant behavior and the infant's performance on perceptual-cognitive tasks as a function of the infant's sex. A total of 189 12-week-old infants and their mothers were observed in their homes during 2 hours of infant awake time. In addition, the Mental Development Index (MDI) of the Bayley Scales of Infant Development was administered to each child in his/her home and the Corman-Escalona Scales of Object Permanence and an attention task were conducted in the laboratory. Results showed that for middle class infants, males received somewhat more proximal behaviors than females while the reverse appeared to be the case for the lower class infants. The highest proportion of maternal interactive behavior, however, was found for mothers of lower SES males, while middle class mothers showed no interactive differences between males and females. Male infants were more responsive to their mother's interactions than female infants; however, there were few sex differences in infant behavior. Contrary to previous findings, males did not fret or cry significantly more than females. On the cognitive measures, a modality difference in habituation was evident on the attention task, but no sex differences were found for the Bayley MDI or object permanence performance. Data indicated a direct association between maternal behavior and infant perceptual-cognitive competence by 12 weeks of age with maternal interactive behavior showing an overall significant relationship and maternal frequency behavior showing different effects for male and female infants. (JMB)