It is demonstrated that the meaning given to an ambiguous word (e.g., "stick") can be biased by the masked presentation of a polymorphemic word derived from that meaning (e.g., "sticky"). No bias in interpretation is observed when the masked prime is a word that is semantically related to the target with no morphological relationship (e.g., "glue"), though such a semantically based bias is revealed when the prime is unmasked. Because the masked priming results cannot be explained in terms of facilitation of processing at either the form level or the semantic level, it is concluded that an intermediate level provides the locus of the effect, referred to here as the "lemma" level. Thus, any model of lexical processing needs to incorporate such an intermediate level to capture the relationship between stem morphemes and their derived forms. (Contains 1 footnote and 4 figures.)