Pelger-Huet's granulocyte abnormality is discussed. This abnormality is characterized by the presence of granuloyctes with a non-segmented nucleus and thick nuclear chromatin gathered in large lumps, in contrast with the "youthful" appearance of the nuclei. In humans, a real Pelger-Huet abnormality with familial character and dominant autosomic transmission is distinguished together with a pseudo-Pelger in which the granulocytes assume cytological features similar to those typical of the constitutional abnormality but whose genesis is closely correlated to the disease of which these are an epiphenomenon.