Utilizing a modified capillary tube technique, analysis revealed a significantly higher mean blood coagulation time in a group of 14 dogs with mastocytoma when compared to the mean coagulation time of 70 non-mastocytoma dogs. In dogs with mastocytoma, approximately one third had intermediate size tumors and a prolonged mean coagulation time and two thirds had a mean coagulation time that did not differ significantly from the mean of the non-mastocytoma dogs. Evidence for heparin release and possible heparin inactivating mechanisms is reviewed.