A female patient with hair cell leukaemia was treated with human leukocyte interferon at a daily dose of 1.5 ? 104 for 27 days. The bone marrow cells were studied with electron microsepy. The follow up investigation of the bone marrow hair cells revealed that there were widening of the perinuclear space, disintegration of nuclear membrane, vaeuolization as well as expansion of the rough endoplasmic retieulum