An experiment is reported on the effects of cognitive similarity on proactive and retroactive interference (PI, RI) in short term memory. To avoid confounding between cognitive and acoustic similarity, the materials - i.e. words and digits - were matched with respect to vowel pattern. Effects of both PI and RI were found on recall of words, while recall of digits was only affected by RI. It is suggested that a string of digits can be more easily organised than a string of unconnected words. If this were true, PI would affect specific associative connections between successive items, whole RI would have a more aspecific disturbing influence with most effect on items of the same category. © 1972.