GF 109203X is a potent and selective inhibitor of protein kinase C, selective for the ?? and ??1 isoforms (IC50 values are 0.0084, 0.0180, 0.210, 0.132, and 5.8 ??M for ??, ??1, ??, ?? and ?? isoforms respectively). Selective over MLCK, PKG and PKA (IC50 values are 0.6, 4.6, and 33 ??M respectively). Potent antagonist at the 5-HT3 receptor (Ki = 29.5 nM).