A dissertation proposal involved a study to observe spontaneous behavior of children in interaction with materials in order to gain understanding of the factors that influence classificatory and imaginative behavior in free play. Children at two levels of ability in terms of classification skills were observed in interaction with materials at two levels of complexity during four play sessions. Materials were two related sets of specially designed blocks defined as simple or complex on the basis of the variations and the possible combinations among individual members of the set. The 3- to 6-year-olds were to play in pairs to simulate the social play typical of children in natural situations. Observation focused on free classification, creative or design behavior, imaginative behavior or variety of themes. A final measure was used to determine the effect of interaction with the materials on classification ability. Results consisted of observational data from the experimental sessions and posttest scores. Non-definitive results indicated replicating the experiment with modifications. It was not feasible to use pairs as the experimental unit, so one child at a time was observed. Interaction between ability and complexity of blocks was found in testing, but not in observed behavior. No interaction effects with age or ability were observed. A revised study is proposed involving age as the independent variable and redesigned materials. (KM)