Subjects were presented with word pairs (e.g., bed-mattress) and timed as they decided whether one word named part of the item named by the other word. Yes responses were facilitated, and no responses were impeded, by relation similarity (i.e., the similarity of the relation between the two stimulus items to the part-whole relation). Item similarity, the similarity of the two stimulus items to each other, had no effect. The results support the view that similarity effects in semantic decision tasks are due to the comparison of elements of the relation between the two stimulus words against relation elements that serve as criteria for the target relation that the subject has been asked to identify.