Phospholipase C (PLC) is shown to comprise at least nine isoforms. These isoforms can be separated into three structurally related classes. Within a class the isozymes have similar enzymological properties. In the case of the PLC gamma class, both isoforms may be regulated by tyrosine phosphorylation. For PLC gamma 1 we show that the tyrosine phosphorylation sites are contained within the SH2/SH3 region or 'modulatory domain'. The overexpression of PLC gamma 1 in Rat-2 cells results in increased phosphatidylinositol breakdown in response to PDGF treatment, demonstrating that PLC gamma 1 mediates this response. We note that thrombin activates PLC gamma 1 in addition to other PLC isoforms.