The regulation of glutamine synthetase (GS) fromAgaricus bisporuswas studied at the posttranscriptional level using a specific antibody fraction directed against purified GS. The cross-reactivity of the antiserum against variousAgaricusspecies and other fungi was tested and low reactivity with the Ascomycetes was found. GS protein and activity levels were measured in cell-free extracts of mycelium grown on different N sources. In mycelium grown on glutamine or ammonium as N source, the biosynthetic GS activity is higher than the transferase activity. Moreover, the results show a correlation between GS biosynthetic activity, GS protein, and previously reported mRNA levels. Also, after addition of ammonium or glutamine to glutamate-utilizing cultures, transferase activity decreased more rapidly than biosynthetic activity and GS protein level. This suggests a conformational modification which only affects transferase activity.