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Haemolytic anaemia was produced in rabbits by the administration of a regular course of incompatible red cell iso‐antibody. A greatly increased rate of red cell production was observed but haemoglobin and reticulocyte counts exhibited vigorous and continuing fluctuations with, in the adult rabbits, a regular periodicity. The effects of steroids and splenectomy on this antibody‐mediated red cell destruction were found to be virtually unobservable, and the implications of this with regard to the haemolytic anaemias of man resulting from red cell destruction by iso‐ or auto‐antibodies are discussed. The significance of the fairly regular oscillations was analysed on the basis of evidence on the various parts of the haemopoietic system. The nature of these and other oscillations observed led to the suggestion that they could be understood in terms of interactions of a four‐component system: the stem cell population; a stem cell specific mitotic inhibitor, termed chalone, which is produced in proportion to the stem cell population and catabolized in proportion to its own level; the differentiated cell population; and erythropoietin. The features of the various oscillations in terms of the four‐component system, and the effects of the parameters on these features, are discussed.