We screened for increased osmotic fragility of erythrocytes in 1464 healthy German blood donors. The osmotic fragility was determined by an acidified glycerol lysis test (AGLT) using glycerol-sodium phosphate-buffered NaCl solution. Since the original test described by Zanella et al. [23] showed only low specificity for hereditary spherocytosis, we used a modification with 0.0093 M sodium phosphate-buffered glycerol-saline solution, pH 6.90, instead of the original 0.0053 M sodium phosphate buffer, pH 6.85. Sixteen of the donors (1.1%) had a "pathologic result," similar to that of 32 patients with hereditary spherocytosis: AGLT 50 less than 5 min ("half-time of AGLT, defining normal and pathologic results). The osmotic fragility of the erythrocytes from 12 of these donors was further investigated using the conventional test with hypotonic NaCl solutions. With one exception, increased osmotic fragility was verified in all of them by both tests. Further hematologic data showed a mild reticulocytosis (2% and 2.6%) in two of the donors. One donor had a moderate reticulocytosis of 6.5%, probably due to a mild, previously undiagnosed spherocytosis; 99 of the donors had an intermediate result (AGLT 50: 5-30 min). Hypotonic lysis of their erythrocytes by the conventional method showed a normal result; there were no signs of increased hemolysis. Thus they are not definitely regarded as having increased osmotic fragility of their erythrocytes. Erythrocyte osmotic fragility shows a wide distribution range in the normal population and might be normally distributed. Thus the blood donors with "pathologic AGLT (less than 5 min)" probably represent only one end of a continuum of salt-dependent hemolysis, and not a separate entity. However, they did show additional minor signs of a functional defect of the erythrocyte membrane and therefore could be carriers of a spherocytosis trait. The frequency of carriers of an erythrocyte membrane defect (possible spherocytosis trait) could be as high as 1.1% in the general population and would distinctly exceed the prevalence of patients with apparent spherocytosis (0.02%).