We have investigated the actions of the PLC inhibitor, U73122, and its close analogue, U73343, which does not inhibit PLC, in Fura-2-loaded human platelets. Rises in [Ca2+]i evoked by thrombin and collagen, and the TxA2-dependent rise in [Ca2+]i evoked by thapsigargin, were abolished by U73122, indicating that it inhibits the activity of both beta and gamma isoforms of PLC. The supposed control compound U73343, was found to inhibit TxA2 formation; it therefore partially inhibited the rise in [Ca2+]i evoked by low concentrations of thrombin, by thapsigargin or by collagen. U73343 had a greater effect than aspirin on the action of collagen, indicating an action on the TxA2-independent component of the signal, via PLC gamma-U73343 lowered TxA2 production by inhibiting the activation of cPLA2, probably at a tyrosine phosphorylation step. U73343 seems to inhibit only the tyrosine kinases involved in the activation of PLC gamma and the generation of TxA2. In contrast, U73122 increased tyrosine phosphorylation of platelet proteins, perhaps by inhibiting receptor independent tyrosine phosphatases, but inhibited all further tyrosine phosphorylation on addition of thrombin or other agonists.