This study describes the initial validation of an innovative social-behavioral observational assessment tool that is designed to be used on a repeated basis to assess growth and development of social competence over time to: (a) identify the social functioning of all students, (b) assist in planning support for students at risk, and (c) evaluate the effectiveness of individual and system-wide interventions. Eighteen first-grade students were monitored over an 8-week period using the Initiation-Response Assessment (IRA) Code. The School Social Behavior Scales, a published teacher rating scale, was included as a criterion measure. Estimates of reliability and criterion-related validity were calculated for the IRA. The measure's sensitivity to growth over time and between-group variability were also assessed using hierarchical linear modeling procedures. Results indicate that scores on this measure are stable, and tap constructs similar to those assessed via teacher rating. (Contains 6 tables and 1 footnote.)