Previous studies have not systematically assessed the effect of fungal biomass on polymorphonuclear leucocyte (PMN)-induced hyphal damage (HD) of filamentous fungi. We hypothesised that fungal biomass is a significant factor affecting PMN-induced HD. One isolate each consisting of a volume of 2 x 10(4) conidia ml(-1) of Aspergillus fumigatus, Aspergillus flavus, Aspergillus terreus, Rhizopus oryzae, Rhizopus microsporus, Cunninghamella bertholletiae, Scedosporium prolificans and Fusarium solani were incubated for six different time periods yielding biomass values between 0.01 and 0.1 optical density (OD, 405 nm). Polymorphonuclear leucocyte were added at effector-target (E : T) ratios of 5 : 1, 10 : 1, 20 : 1, 50 : 1 and 100 : 1, and HD was assessed by XTT [2,3-bis-(2-methoxy-4-nitro-5-sulphophenyl)-2H-tetrazolium-5-carboxanilide] metabolic assay. Hyphal damage decreased with increasing biomass following the sigmoid (E(max)) model (median R(2): 0.87). Hyphal damage at 0.01 OD exceeded HD at 0.1 OD (P < 0.01) by >twofold in 64 out of 80 comparisons. The sigmoid curves were shifted to the right with higher E : T ratios; the EC(50) values (OD values showing HD halfway between maximal and minimal HD) obtained for 50 : 1 or 100 : 1 were higher than for 5 : 1 (P < 0.01). Using the same E : T ratio, interspecies differences were observed; for 5 : 1, lower EC(50) values were obtained for A. flavus and the zygomycete species. In conclusion, PMN-induced HD decreases with increasing biomass. This effect is both species-dependent and E : T ratio-dependent.