The aldehyde forms of vitamin B6, pyridoxal and pyridoxal 5'-phosphate (PLP) have aroused interest as antisickling agents because of their ability to modify hemoglobin (Hb) and their low toxicity. To study their rate of formation and stability inside red cells, pyridoxal-Hb and PLP-Hb were measured in lysates from treated normal and sickle erythrocytes using isocratic high pressure liquid chromatography on Bio-Rex 70. The validity of this assay was confirmed by isoelectric focussing, fluorescence scans of reduced globin, and treatment of cells with pyridoxal 14C. Optimal conditions were described for treatment of whole blood with pyridoxal and washed erythrocytes with PLP. Although there was competition between 2,3-DPG and PLP, but not pyridoxal, for binding to Hb, depletion of 2,3-DPG prior to treatment was unnecessary. No special requirements were noted for the anticoagulants or buffers used. Sickle erythrocytes formed PLP-Hb more rapidly than normal erythrocytes, but pyridoxal-Hb appeared at the same rate in both types of erythrocytes. During incubation of treated erythrocytes in untreated plasma, the stability of pyridoxal-Hb varied inversely with the hematocrit, but PLP-Hb was stable at all hematocrits tested. The absence of hemolysis during a 4 day incubation of treated normal red cells implies that treatment with pyridoxal or PLP did not severely impair red cell metabolism.