The conidiation process of Neurospora crassa is characterized by three morphogenetic events: adhesion of hyphae, development of aerial hyphae, and conidia formation. At the onset of all three events a spontaneous, low-level chemiluminescence was detected, indicating the formation of reactive oxygen species. Hyperoxic conditions increased chemiluminescence and accelerated differentiation. Hypoxic conditions abolished both chemiluminescence and differentiation. Chemiluminescence was enhanced by lucigenin and/or luminol. Butylated hydroxytoluene and antioxidants that do not readily enter the cells, like superoxide dismutase and catalase, did not lower the chemiluminescence nor had they an inhibitory effect on the differentiation process. In contrast, N,N'diphenyl-1,4-phenylene diamide, 1,3-dimethyl-2-thiourea, ammonium pyrrolinedimethyl-dithiocarbamate, and N-acetyl-L-cysteine retarded the onset or abolished both the chemiluminescence and the differentiation process. These results further support our hypothesis (Hansberg, W.; Aguirre, J. J. Theor. Biol. 142:201-221; 1990) that a hyperoxidant state triggers cell differentiation events.