An interactive activation model for picture naming was used to guide treatment of a semantic-level deficit in 4 individuals (aged 47-59 yrs) with aphasia and severe picture-naming problems. Ss exhibited a profile consistent with Broca's aphasia with severe naming deficits, part of which was attributable to a semantic impairment based on testing of the lexical system. A semantic-based treatment was used to train naming of nouns in 2 semantic categories using a single-participant multiple baseline across behaviors and participants. Additional treatment, which included orthographic and phonological information about target words, then was applied. Treatment responses and error patterns demonstrated that semantic treatment resulted in improved naming of both trained and untrained items for 2 of 4 Ss. Two Ss did not show improved naming until treatment emphasizing the phonological form of the word was provided. This study demonstrates the utility of using an interactive activation model to plan treatment based on levels of disruption in the lexical processing system. Stimuli from Set A and Set B are appended.