An enzyme-linked immunosorbent assay (ELISA) was used to detect a sap-staining fungus, Ophiostoma piceae, and a biological-control agent, Gliocladium roseum, grown in liquid culture and in wood. A polyclonal serum prepared against whole cell fragments from broken mycelia of O. piceae detected O. piceae in liquid culture at 0.25 μg dry weight/mL; however, there was moderate cross-reactivity with G. roseum. Antiserum adsorbed on G. roseum had almost no reactivity with G. roseum but still reacted strongly with O. piceae. The specificity of these sera was verified, and the antigenic sites were localized, by immunogold labeling and electron microscopy. These studies confirmed that the adsorbed serum could differentiate between G. roseum and O. piceae and showed that the cell wall was the most reactive cellular component. These results are discussed in relation to the development of immunological probes for the detection of sap-staining and biological control fungi. Key words: polyclonal serum, enzyme-linked immunosorbent assay, immunogold labeling, sap-staining and biological control fungi, electron microscopy.