Interleukin 3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 5 (IL-5) exert their biological activities through interaction with cell-surface receptors that consist of two subunits, a specific a subunit and a common beta transducing subunit (beta c). We have evaluated the effect of growth factors on the expression of beta c in normal monocytes. Addition of either GM-CSF or M-CSF to monocytes elicited a marked increase of beta c chain expression, a phenomenon seemingly related to a stimulation of the transcriptional activity of this gene mediated through an enhancement of the PU.1 DNA binding activity. Interestingly, during the activation of beta c chain expression by growth factors a switch from the synthesis of the truncated betaIT to the full-length beta c was observed. Similar observations have been made also in the growth factor-dependent erythroleukemic cell line TF-1, showing that GM-CSF deprivation elicited a marked decrease of beta c chain expression.