Separation and greeting responses were examined in a longitudinal study of 26 infants, aged 15 to 54 weeks, observed at home. All instances of a person's leaving and entering the room during home visits were observed. The developmental trend--both onset and relative frequency--of each separation behavior was plotted at three-week intervals. The onset of these behaviors was found to be earlier than most other investigators have reported. Following the mother upon her departure and greeting her positively upon her return were found to be more frequent than crying upon separation. Following showed the only sex difference; boys followed more than girls. Crying was more frequent when the infant was left confined rather than free to move about. Separation protest was differential to the mother from its onset, whereas following was not. Most positive greeting behaviors were differential, but smiling was not, seemingly serving affiliative as well as attachment functions. Infants when left totally alone are more likely to exhibit separation-related behaviors than when left with companions; this finding is interpreted in the light of Bowlby's ethological-evolutionary model of social development. (Author)