A rosette assay was developed for the detection of haem receptor‐bearing cells. Indicator particles were prepared by covalent binding of haem to acrylic microbeads. The new method was tested on K562 human erythroleukaemia cells, known to take up haem. In tests on several batches, 80–90 of the K562 cells were rosetted with haemmicrobeads whereas mature erythrocytes were haem receptor‐negative. Rosette formation was inhibited in a dose‐dependent manner by micromolar concentrations of free haemin but not by albumin or transferrin. Uncoated microbeads or albumin‐coated microbeads did not attach to K562 cells but transferrin‐coated microbeads rosetted 50–70 of them. Diferric transferrin inhibited these rosettes, but haemin
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