When participants are required to respond to a target letter imbedded in a stream of rapid serially presented letters, perception of a 2nd target letter is impaired if the interval between the 2 targets is less than about 450 ms. This attentionally based posttarget suppression in visual processing, referred to as the attentional blink (AB), is not found when there is a brief pause in the stream immediately after the 1st target. To investigate the importance of posttarget stimulation in AB production, the categorical, featural, and spatial similarity of the immediate posttarget item to other items in the stream was manipulated. Although featural and spatial dissimilarity produced significant attenuation of the AB effect, categorical dissimilarity did not. Significant AB effects were found in all conditions, suggesting that the presentation of any patterned stimulus in close temporal proximity to the target provokes the AB.