The study examined the effectiveness of enhancing perceptual differentiation in the training of four developmentally delayed preschool children who were so low-functioning that they did not demonstrate oddity responding (ability to choose one distinct stimulus from a group of identical stimuli). Instead of the Arabic numerals used in the original study, subjects were provided with a specific line drawing surrounded by eight other identical line drawings. Drawings were of common objects. Under these conditions the previously unsuccessful children demonstrated statistically significant increases in percentage of correct oddity responses immediately upon introduction of the arrays of familiar stimuli. Results suggest that enhancement of perceptual differentiation can facilitate relational learning of the type required by the oddity task. (Author/DB)