The conclusion by Suit, Matney, Doudney & Billen (1964) that Hfr donor cells of Escherichia coli K12, starved of required amino acids can mate, has been re-examined. It appears that their conclusion is not valid and that apparent fertility of amino-acid starved cells is due to cross-feeding by the F− cells. The relationship of this result to the alternative mechanisms for chromosome transfer in E. coli is discussed.