The effects of 2 different types of positive and negative instances in learning "non-dimensioned" concepts were investigated. "Non-dimensioned" concepts are defined only by the presence or absence of distinct attributes. One-hundred-seventy-one 8th graders were given series of all positive or alternating positive and negative instances. Series of positive instances in which successive instances shared the same irrelevant attributes and series of positive and negative instances in which the negatives lacked several attributes when compared to a preceding positive resulted in poorer performance (p< .01) from Ss than did series of positives in which the instances shared none of the same irrelevant attributes and series of positives and negatives in which the negative instances lacked only one attribute when compared to a preceding positive instance. The results agreed with findings with "dimensioned" concepts and indicated the Ss may be using negative instances to generate hypotheses of possible relevant attributes. (Author)