IN THIS STUDY the extent to which morphological generalization can account for increases in vocabulary size was investigated. Focusing specifically on morphological generalization with derivational suffixes, the study tested students' ability to use morphological and contextual information to determine the meaning of unknown words. Students in the fourth, sixth, and eighth grades were randomly assigned to receive training in one of two word sets. Later they were posttested, in weak and strong contexts, on words that were morphologically related to both sets of training words. Subjects' success in deriving the meaning of unfamiliar words was affected by prior experience with related words and by the strength of the surrounding sentence contexts; sixth- and eighth-grade students were more skilled than fourth-grade students in using context clues as well as morphological clues. However, subjects did not combine these two information sources to yield higher vocabulary scores than obtained with either source by itself. Estimates of the amount of morphological generalization varied widely, depending upon which of two scoring procedures was used.