Human myeloid leukaemia (U-937 and HL-60) cells when incubated at low cell densities with human recombinant gamma-interferon underwent functional maturation without any loss of proliferative potential relative to uninduced cells. In addition, the proportion of cells in S,G2/M and levels of c-myc oncogene (mRNA and protein) were maintained at the same level as those of untreated control cells. However, cells grown under similar conditions but with retinoic acid matured to the same extent but became growth inhibited with concomitant reductions in the proportion of cells in S,G2/M and levels of c-myc mRNA and protein. These studies indicate firstly that c-myc levels are regulated independently from differentiation in myeloid (non lymphoid) cells, secondly that gamma-interferon can induce differentiation without growth arrest under conditions of low cell density and thirdly emphasise the close association of c-myc expression with proliferative capacity.