A series of 1 (2H)-isoquinolones was prepared by reaction of 4-acylisocoumarins with ammonium carbonate or primary amines in acetic acid. These compounds, after a 15-min pretreatment, inhibited histamine release from isolated rat mast cells, and some of them were potent when tested on rat passive cutaneous anaphylaxis (PCA assay 3 h after i. p. injection). The activities of these compounds were, however, lower than that of the already known 4-(4-carboxybenzoyl) isocoumarin.