1. Previous studies have shown that leukaemic plasma inhibits sodium efflux from erythrocytes. In this study the effect of an extract of myeloid leukaemic blast cells on sodium efflux was investigated.

2. Blast cell extract decreased the sodium efflux rate constant in erythrocytes from 0.395 ± sem 0.028 to 0.310 ± 0.018 (P < 0.02), whereas a leucocyte extract had no significant effect.

3. The extract had an inhibitory effect after heating at 80° C for 30 min, and in the presence of ouabain and frusemide alone or in combination.

4. These studies suggest that myeloid leukaemic blast cells contain a heat stable factor which inhibits the efflux of sodium at least in part through a pathway which is not inhibited by ouabain or frusemide.

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