The feasiblity of an observational rating scale which would provide objective, reliable and quantifiable measures of social and affective behavior was investigated. The Observation of Socialization Behavior (OSB), focusing on peer-group behavior of pre-school children and designed for use in unstructured situations was developed. Two forms of the instrument, one for use with videotaped interaction sessions, and the other for live classroom situations, were developed. A combination time-and event-sampling procedure was used and behavioral events were recorded at twenty-second intervals. Observers were trained by videotaped interaction segments, and computation of observer reliability is explained. Investigations provide favorable indications of the validity of the instrument for measuring peer-group behavior of pre-school children. It is concluded that the instrument is useful, flexible, and applicable to early childhood psychometry. (LR)