LY 303366, an inhibitor of 1, 3-beta-D-glucan synthase, was tested alone, or in co-culture with neutrophils or monocytes, for antifungal activity against Aspergillus fumigatus using the XTT metabolism assay. LY 303366 at 0.1 mg/L for 48 h significantly inhibited growth by conidia in a microtest plate XTT assay system. Inhibition was similar if the drug was removed after only 24 h. Microscopically this correlated with less growth and stunted malformed hyphae. LY 303366 (0.1 mg/L) also inhibited the further growth of germlings (43%) in a 24 h assay. Antifungal activity of neutrophils against 24 h control hyphal growth was limited at an effector: target ratio of 400:1. In co-cultures of neutrophils plus drug with hyphal growth from 24 h LY 303366 cultures the antifungal activity was additive. Neutrophils had a similar additive effect even if the drug were not present (i.e. when germinating conidia were pretreated with drug). Under conditions where monocytes did not have significant antifungal activity against hyphae, they collaborated with LY 303366 for significantly increased inhibition from 38% by LY 303366 alone to 67% by co-culture. Thus, LY 303366 has activity against germinating or germinated conidia of Aspergillus, human effector cells act co-operatively with LY 303366, and LY 303366 can sensitize germinating conidia for damage by host cells.