A new whole-blood flow cytometric method has been developed for counting and sizing platelets in samples from cats, a species in which platelet and red blood cell sizes overlap significantly. The method is a modified version of the two-angle laser light scattering technology used by Bayer H*System hematology analyzers. The new method provided accurate platelet counts and mean platelet volumes (MPV, fl) for cats. The method also measured mean platelet component concentration (MPC, g/dl), a parameter which was shown to be sensitive to platelet activation state, and which decreased in value as activation progressed.