A hereditary anomaly of neutrophil granulation in purebred Birman cats was described with respect to genetic, electron microscopic, histochemical, and functional characters. The trait was inherited in an autosomal recessive manner and was prevalent in the population studied. Affected cats had fine eosinophilic granules in the cytoplasm of neutrophils. The granules had normal morphology as determined by electron microscopy and did not stain for acid mucopolysaccharide. Bactericidal activity, phagocytosis, and oxidative function of affected neutrophils were not different from those of unaffected neutrophils. The anomaly was concluded to be an alteration in the content of lysosomal granules with increased affinity for acidic dyes.