The translocation t(8;16) (p11;p13) was found as the sole deviation from the normal karyotype in three patients with acute monocytic leukaemia. The bone marrow morphology was strikingly similar in the two cases where smears were available for re-evaluation: the leukaemic cells showed signs of differentiation, and active erythrophagocytosis was a particularly conspicuous feature. We suggest that t(8;16) (p11;p13) represents a new consistent abnormality in acute monocytic leukaemia, specifically associated with the differentiated subtype (M5b) and with pronounced phagocytic activity by the leukaemic monocytes.