Investigated were the effects of two types of stimuli, preferred and non-preferred, on the free recall and clustering of 16 moderately and severely retarded children (mean IQ = 39.12, mean CA = 10.84 years) who were enrolled in public schools. An empirical determination was made as to whether to classify a particular conceptual category of stimuli as preferred or non-preferred. All Ss were presented with 18 objects from six conceptual categories in blocked condition. Both the preferred and non-preferred stimuli were given to each S in a counterbalanced order using a repeated measures design. The major finding was that the use of preferred stimuli had a significant facilitating effect on recall but not clustering. (Author)