Ergosterol was determined as a chemical indicator of fungal biomass in wheat (w), barley (b), oats (o), and grain corn (c). In samples selected by microscopy and odour as an indicator of good mycological quality the following contents (mg/kg dry matter) were found: 1.96 +/- 0.72 (w), 2.43 +/- 0.82 (b), 2.42 +/- 1.4 (o), 0.61 +/- 0.37 (c). In cereals from farms in Baden-Württemberg (FRG) which were not selected by microscopy and odour 5.8 +/- 3.8 (w), 5.8 +/- 2.0 (b), 15.8 +/- 9.0 (o) mg/kg (year of harvest: 1984); and 3.5 +/- 1.9 (w), 3.9 +/- 1.4 (b), 9.9 +/- 3.6 (o) mg/kg (year of harvest: 1985) were found. Differences between years of harvest, between samples without and with selection, and between oats and the two other types of cereal are significant. Elevated ergosterol contents (as compared to those of selected samples) were found in 66 and 22% (w), 70 and 15% (b), 95 and 86% (o) of farm samples from 1984 and 1985, respectively. The more prolific mold growth in 1984 was mainly due to readier development of field fungi and can be explained by heavier rainfall. Almost all of the ergosterol contents described in literature for wheat and grain corn are in the range found in the present study.