Five murine hybridoma cell lines secreting monoclonal antibodies (MAbs) with similar binding specificities were raised to extracts from cyclamen leaves colonized with Ulocladium atrum, a saprotrophic fungus used for biological control of Botrytis cinerea. One of the cell lines produced a MAb, UA–PC3, which was used to develop a quantitative ELISA to determine the biomass of U. atrum in extracts from colonized leaves. The antigen, which is water soluble, was present on the surface of conidia and hyphae and was secreted into culture fluids. Production of the antigen was much greater in vivo than in vitro. Germination of conidia took place in saline buffers at 4 °C after 6 h. To avoid amplification of the antigen in biomass assays of the fungus in extracts from sporulating tissues, coating of microtitre wells in ELISA was reduced to 1 h.