AbstractThe work reported here clearly demonstrates that a specific growth factor, interleukin-3 (IL-3), which acts on multipotent haemopoietic stem cells as well as on committed myeloid progenitor cells of different lineages (Schrader, 1988; Whetton and Dexter, 1986), specifically induces a modification of the physical state (“fluidity”) of the cell membranes of two IL-3–responsive and apparently normal haemopoietic cell lines. Furthermore, in a derived IL-3 independent myeloid leukaemic cell line, no such physical response to IL-3 binding was observed. The rapidity of the“normal”response suggests further that it may be associated with, or even constitute per se a critical early effect elicited by IL-3 in sensitive cells, the necessity for which is abrogated in the malignant derivative.