Ancylostomiasis in susceptible dogs is accompanied by the development of a microcytic, hypochromic (iron deficiency) anaemia. In the present study, anin vitrocorrelate of cell-mediated immunity (lymphocyte transformation following stimulation with phytohaemagglutinin) was used to monitor the development and maintenance of immune responsiveness during the course ofA. caninuminfection. The results showed that lymphocyte function (as assessed by phytohaemagglutinin reactivity) was significantly impaired during the later stages of infection and that this impairment was very closely correlated with the development of an iron-deficient anaemic state.