Recharge by mafic magma is established as a trigger of major explosive volcanic eruptions. However, recharge by silicic magma can also induce perched resident magma to erupt. The evidence of such recharge may be more cryptic than for mafic recharge and requires detailed petrologic and geochemical study to identify. We present a study of plagioclase petrology supported by other data that implicates silicic-silicic magma interaction during the eruption and evolution of the A.D. 1600 eruption of Huaynaputina, one of the largest historical eruptions. Recharge by silicic magma is probably more common than recognized and should be considered an important eruption trigger.