3 concept-learning experi-ments were performed to determine the effect of permitting stimulus instances, first presented on earlier trials, to remain available for S''s inspection. After the presentation of each stimulus, S stated his guess of the concept. For independent groups of Ss, different numbers of stimuli were available on each trial. In Experiment I, S''s performance improved with increases in number of available stimuli, more markedly in problems of greater complexity. The outcome was similar in Experiment II except that performance worsened slightly when more than 5 stimuli were exposed on each trial. Results on Experiment HI indicate that this interference effect is due to a limit on response-time interval. The major effect of stimulus availability in concept learning was the reduction of memory errors.