A case of acute monocytic leukemia (AMoL) studied by cytologic, histologic, cytochemical, ultrastructural, and cytogenetic technics is reported. The immature monocytes were characterized by strong fluoride-sensitive nonspecific esterase activity. Nuclear irregularity, prominent nucleoli, erythrophagocytosis, and pinocytosis were observed on transmission electron microscopy, whereas broad-based ruffles, similar to those found on normal monocytes, were identified with scanning electron microscopy. Cytogenetic analysis showed an 8/9 translocation and four or five identical 8p- marker chromosomes in most bone marrow cells.