Abstract A major protein was isolated from sclerotia of Sclerotinia sclerotiorum, Sclerotinia trifoliorum, and Sclerotinia minor which comprised ca. 38, 27, and 31% of the total cellular protein, respectively. These proteins were not detected in vegetative cells. The three proteins were similar but not identical. All three proteins reacted equally well with antibody to the S. sclerotiorum protein but they could be distinguished from one another by two-dimensional gel electrophoresis. Polyacrylamide gel analysis of peptide fragments formed by cyanogen bromide treatment also indicated small differences between the three proteins. The finding of these proteins is important because it indicates that fungi, like many higher eukaryotes, have the potential to produce developmentally regulated proteins in large quantities.