Large platelets (megathrombocytes) are young and generally more active than normal sized platelets. Conversely, the presence of predominantly small platelets (microthrombocytes) in canine blood appears to be a specific indicator of immune-mediated thrombocytopenia (IMT). This conclusion is based on data from 68 thrombocytopenia cases (Northern, J. Jr. and Tvedten, H.W. JAVMA 200: 368-376, 1992). Small platelets (mean platelet volume < 5.4 fl) were found in 17 of the 31 IMT cases in this group, but only in 1 of the remaining 37 dogs.