Growth hormone (GH) and prolactin (PRL) quality as lymphohaemopoietic growth and differentiation factors, and so does insulin-like growth factor (IGF)-I, which mediates many of GH activities. Although there is only limited evidence that endocrine, paracrine or autocrine GH or PRL play a role in human leukaemia and lymphoma, the expression of these factors or their receptors may have diagnostic or therapeutic implications. Indeed, the participation of GH, PRL or IGF-I in the development or progression of certain haematological malignancies or to the antitumour immune response has been documented. Examples discussed in this review include a rat lymphoma in which the PRL receptor acts as an oncogene; the rat Nb2 lymphoma, which is dependent on PRL for growth; and experiments showing that PRL stimulates natural killer cell activity and the development of lymphokine-activated killer cells.