The microhematocrit was used as a screening procedure for the identification of sickle cell anemia (SS disease). Transillumination of the packed cell layer in the hematocrit tube reveals an upper translucent layer of nonsickled cells and a lower opaque layer of sickled cells in 75% of patients with homozygous SS disease (positive sicklecrit). When the sicklecrit and the sodium metabisulfite screen are both positive, there is presumptive evidence of homozygous SS disease. No positive sicklecrits were found in children who did not have homozygous SS disease. The finding of a positive sicklecrit appears related to the number of sickled erythrocytes on the peripheral blood smear and, perhaps, inferentially, to in vivo sickling.