Preliminary results are presented for an instrument to measure social behavior of preschool children, focusing on cooperation. Cooperation is defined either as behavior for the joint gain of the participants in an interaction or behavior in which each child attains his individual goal. The Oregon Preschool Test of Interpersonal Cooperation (OPTIC System), a situational test which permits systematic observations of behavior in realistic contexts, elicits cooperative responses and permits social behavior to be quantified. Several conclusions are drawn: 1) The tendency for children to cooperation varies with situation; 2) The behavior of each member of a pair cannot be treated as statistically independent; 3) Cooperation defined as a complex, heterogeneous concept accounts for a small but relatively stable amount of variance across situations; 4) The data indicate reasonable internal stability of the measure; and 5) Relationships between age, sex, and frequency of cooperative responses were noted. (LH)