Two expts. were performed to determine the influence of set upon a phenomenon which has been described as perceptual defense. The expts. involved tachistoscopic presentation of taboo, taboo-similar, and neutral words. The variables investi-gated were: (a) the effect on instructions preparing S for taboo words, and (b) the effect of preparing S for taboo words but presenting taboo-similar words. The Ss were male undergraduate students matched on the basis of a practice day performance. The measures used were recognition thresholds (time) and prerecognition hypotheses. The data support the following con-clusions: (1) Conditions producing apparent perceptual defense may be manipulated by varying the instructions given to S. Where S is set to expect taboo material, perceptual defense does not occur; (2) Where S is set to expect taboo words, a sensitiza-tion to these words occurs. Where S is not originally set to expect taboo words, a habituation effect to successive taboo words occurs; (3) The Ss may be induced, through set, to report taboo prerecognition hypotheses to words similar to taboo words with a subsequent "vigilance" effect to the following words. It was suggested that the results provide a more direct explanation of the data which have previously been interpreted in terms of a mechanism of perceptual defense. Specifically, a perceptual principle of greater generality which would relate the ordinarily increased recognition thresholds of taboo material to the domi-nance of alternative hypotheses or sets was proposed.