1. Previous studies have shown that leukaemic immature cells, specifically promyelocytes but not mature leucocytes, contain and release an inhibitor of ouabain-insensitive sodium transport (inhibitin).
2. In the present study, medium from cultured leukaemic promyelocytes significantly reduced the ouabain-insensitive sodium efflux from erythrocytes, whereas medium from the same cell line which had been made to differentiate did not have this effect.
3. Culture medium from normal bone marrow cells (containing promyelocytes) also significantly (P < 0.001) reduced ouabain-insensitive sodium efflux.
4. These data suggest that inhibitin is secreted by primitive but not by mature leukaemic cells and normal bone marrow cells.