PMNL’s are marginated in numbers on the endothelium of the peripheral blood vessels and are released into the circulation in response to stress, infection and inflammation. PMNL values are often reported as the percentage of neutrophils (N %) in a differential cell count. When reported in this way, the neutrophil:lymphocyte ratio in mature adult elite horses is somewhere around a 60%:40% ratio, respectively. Neutrophilia and neutropaenia are better identified by recourse to absolute PMNL counts. Neutrophilia, when accompanied by hyperfibrinogenaemia, is usually noteworthy when PMNL’s are > 7.0 x 109 / L. It is impossible to meaningfully differentiate between the neutrophilia of stress and that of infection and inflammation, without concurrent measure of acute phase proteins such as fibrinogen. Neutropaenia can be defined as PMNL’s < 2.0 x 10 9 / L, when they can be a very disturbing indicator of impending catastrophies, e.g. colitis / laminitis / peritonitis, etc. Immature neutrophils, whether toxic or band form, are associated with the presence of pus-forming infections and are best evaluated using manual differential counts derived from fresh blood smears.