A process for detecting parasites which are found in the blood of a host. A sample of blood is drawn into a capillary tube which contains a generally cylindrical mass having a specific gravity such that it will float in one of the cell layers when the sample is separated by centrifugation. The specific gravity of the mass is selected so as to cause the cylindrical mass to combine with the bore wall of the capillary tube to form a thin annular space in the capillary tube into which the parasites will be crowded, thus increasing the concentration of parasites in a restricted area of the centrifuged blood sample and rendering the parasites highly visible. A stain may be used to differentially color the parasites. One specific parasite which can be detected in this manner quickly and inexpensively is heartworm.