Recombinant human interleukins 1 and 2 (rhIL-1 and rhIL-2) have recently been shown to interact with cells not only of the lymphoid lineage but also of myeloid origin. In fact, rhIL-1 indirectly stimulates normal haemopoietic cell proliferation without any influence on the differentiation, and can also induce lysozyme secretion by leukaemic cells. On the other hand, rhIL-2 is capable of indirectly stimulating the proliferation of monocytic precursors. Nevertheless, the effect of both factors on the expression of Fc receptors or on the proliferation of cells of myeloid origin has not been evaluated thus far. In order to assess whether these interleukins take part in the induction to proliferation of Fc receptor expression by homogeneous populations of myeloid cells, two leukaemic cell lines were used in this work, namely, WR19M.1, macrophagic, and WEHI3BD-, myelomonocytic. The results achieved show that: 1) rhIL-1 strongly stimulated the expression of Fc receptors in both leukaemic cell lines; 2) rhIL-1 does not stimulate the proliferation of these lineages; 3) on the contrary, rhIL-2 stimulated cell proliferation in both leukaemic cell lines, and 4) rhIL-2 had no effect on the induction of Fc receptors or on the stimulation of lysozyme secretion. The role played by the proliferative effect of rhIL-2, as well as the rhIL-1-mediated appearance of Fc receptors, in myeloid proliferation and differentiation are discussed.