Human erythrocytes extensively crosslinked with glutaraldhyde display very high mechanical and chemical resistance, but still bind oxygen reversibly. Depending on the initial reaction conditions the oxygen affinity of fixed erythrocytes is either high or (partially) low, the binding curve being always non cooperative. Cells which are extensively crosslinked but still active may find a number of biotechnological applications; fixed erythrocytes may (for example) be investigated as possible blood substitutes.