A 17-yr-old black male with hemolysis and pigmenturia but no anemia was found to have hemoglobin Köln (α2β298 val→met [FG5]). Splenectomy was done because of complicating thrombocytopenia. Thrombokinetic studies with 51Cr tagged platelets suggested hypersplenism, and after surgery the platelet count returned to normal. The red cell t ½ 51Cr was more than doubled, but the red cell life span (DF32P) was more modestly improved (30.6 → 47.2 days). The "elution" of 51Cr from the red cells presplenectomy was 5.6%/day, whereas after surgery it was normal (1.9%/day), accounting for the disparity between the survival methods. Study of the isolated cyanferri derivative of hemoglobin Köln by ultracentrifugation at various salt concentrations and various pH’s indicated an increased tendency to dimer formation under conditions where normal hemoglobin is a tetramer. This results from the site and type of amino acid substitution and accounts in part for its instability.