A survey of assessment procedures of the newborn and of the infant during the first month of life was conducted; the survey indicated that there were instruments for evaluating the newborn and for evaluating the four-week-old infant, but there was no single procedure which included an evaluation of both the newborn and the four-week-old infant. This study is concerned with trying to understand individual differences in infant behavior which can be used to specify the dimensions and parameters of an effective environment for particular infants. Reported is work involving a sample of 60 infants, 30 males and 30 females, who were each tested at three or four days of age in the hospital and then retested four weeks later in the home. Interest is primarily in the stability of performance over the four weeks and secondarily in the distribution of scores at both ages and in sex differences. Subjects include mainly white, upper lower, middle, and upper middle class infants, all of normal birth-weight with Agpar scores at five minutes well within the normal range. For the retest at approximately four weeks of age, the mean age for females was 27.87 days with a range of 24 to 33 days; for males, the mean was 27.79 days with a range of 24 to 34 days. After wheeling the infant into the examining room, his initial state was observed to two minutes. Then the pen light flashlight was flicked across the closed eyes and any response observed. Female scores were generally more stable than males over the four-week period. Males showed significant shifts, for items measuring peak of excitement, alertness, following with head and eyes, reaction to sound, and pull to sit. Data tables and charts are provided. (CK)