The central question of this report concerned the role of formal similarity in free recall of lists of trigrams and lists of three-letter word triads. Similarity was manipulated among trigrams by duplicating words. An initial study showed that lists of 16 letters were learned more rapidly than a list of 16 three-letter words. Therefore, in the major experiment, the subjects were given all appropriate elements on test trials so that only associative learning was required. Increases in formal similarity caused decreases in rate of learning for both types of lists and the mechanisms of the interference seemed to be the same for both types of lists. However, the learning of the trigram lists was more rapid than the learning of the triad lists, the difference being maximal with low similarity. (Author)