1. At a high dilution in Ringer solution (1:200), leukaemic plasma significantly (P < 0.05) decreased the ouabain-sensitive and increased ouabain-insensitive components of sodium efflux from erythrocytes. At a low dilution (1:10) leukaemic plasma predominantly decreased the total and ouabain-insensitive component of sodium efflux (P < 0.01).
2. Erythrocytes from patients with leukaemia had a high affinity for the plasma factor which inhibited the total and ouabain-insensitive efflux (inhibitory factor).
3. Washings of leukaemic erythrocytes which had been incubated in leukaemic plasma contained a factor which significantly decreased the ouabain-sensitive and increased ouabain-insensitive components of sodium efflux (the anti-ouabain-like factor).
4. These studies show that leukaemic blood contains two factors which have opposite effects on sodium efflux from erythrocytes. These factors may contribute to the high incidence of multiple electrolyte disturbances in acute myeloid leukaemia.