An antiserum raised against a purified tobacco beta-1,3-glucanase (PR-N) was used to study the subcellular localization of enzyme in fungus-infected plant tissues by means of post-embedding immunogold labeling. In susceptible tomato plants, the enzyme accumulation was found to occur as a result of successful tissue colonization, whereas it appeared to be an early event associated with limited spread of the fungus in resistant tissues. Although marked differences between susceptible and resistant tomato cultivars were observed in the rate of production of beta-1,3-glucanase, the pattern of enzyme distribution was similar. The enzyme was found to accumulate predominantly in host cell walls and secondary thickenings of xylem vessels. By contrast, a very low amount of enzyme was associated with compound middle lamellae. The occurrence of beta-1,3-glucanase at the cell surface of invading fungi was an indication of their possible antifungal activity. A low enzyme concentration was detected in vacuoles of both healthy and infected tissues. In infected eggplant tissue, the pattern of beta-1,3-glucanase distribution was similar to that observed with tomato. Whether these hydrolases accumulate first in vacuoles and are subsequently conveyed toward the outside to participate in fungal wall lysis remains to be determined.