A series of 1-phenylisoquinolines and related compounds was prepared and tested for potential antiallergic activity. Several compounds of this series inhibited the antigen-induced wheal formation in rat passive cutaneous anaphylaxis (PCA) assay, a commonly used test for antiallergic activity. Many of these compounds also inhibited the antigen-induced histamine release from passively sensitized guinea pig lung slices. Furthermore, almost all of these derivatives inhibited the cyclic nucleotide phosphodiesterase, suggesting this as one of several possible mechanisms of action.