Recent work suggests that working memory span (WMS) tasks are not simple measures of the capacity to simultaneously store and process new information. Instead, these measures may be influenced by numerous factors, including proactive interference (PI). The current study examined whether WMS, like other memory tasks, is also influenced by PI from prior memory experiments. Experimentally experienced and naive participants completed a speaking span task. Span scores were lower for experienced than for naive participants, but other cognitive scores were not. In combination with other work, these results suggest that WMS estimates are not pure measures of capacity and may be partially determined by PI.