In this study an attempt was made to measure variables which were expected to be related to individual differences in infants' social responsiveness toward strangers. The subjects were 48 infants (24 boys and 24 girls) between 8 and 13 months of age. Most were children of Cornell University faculty or graduate students, but a broad range of families from a small city in upstate New York was represented. Before an infant was tested for stranger response, his mother was interviewed in her home. Interview questions were designed to provide measures pertinent to five general hypotheses about determinants of individual differences in reactions to strangers. These hypotheses were: (1) Infants who have had little exposure to people would be more likely to be upset by strangers. (2) Babies who protest separation from mother would be more likely to be upset by strangers. (3) Strangers who are visually quite similar to the appropriate sexed parent would be more upsetting to the baby because of cognitive confusion on the baby's part. (4) Infants who are irritable or sensitive would be more likely to be upset by a stranger. (5) Infants whose mothers are protective about strangers would transmit their concern to their infant. A few days after each interview, systematic observation was made of each infant's reaction to two unfamiliar adults. Detailed behavioral descriptions of the reactions were recorded. (CS)