A laticifer is an elongated tubular cell and its cytoplasmic content (latex) is thought to be involved in defense against herbivores and microbes. Previous studies investigated laticifer transcriptomes in the unlignified and lignified tissues of mulberry (Morus alba) but protein accumulation in laticifers in unlignified and lignified tissues is poorly understood, except the conclusion of a previous study that insecticidal chitinase-like proteins (LA-a and b) were abundant in the laticifers of unlignified tissues while antifungal class I chitinase (LA-c) was abundant in the laticifers of lignified tissues. In order to understand precisely the physiological roles of laticifers in these tissues, this study identified the major proteins in them using mass spectrometry and Edman sequencing after separation by ion-exchange chromatography and SDS-PAGE. In addition to LA-a, b and c, this study has shown that mulberry laticifers accumulate large amounts of biotic-stress-related defense proteins, e.g., pathogenesis-related protein-1, β-1,3-glucanase, class V chitinase, osmotin and lectins. The abundance of some proteins varied among the laticifers of unlignified and lignified tissues, which suggested that the laticifers may have adapted to different threats.