Heinz body formation was investigated and compared in the blood of prematures, full-term infants and adults. Factors which can be demonstrated to increase rate of Heinz body formation include constant aertion at 37[degree]C and dilution of the sample, while N atmosphere and cold inhibit it. Electron microscope observations demonstrate changes in cell contents and in membrane when Heinz bodies are formed. Comparison with the blood of adults reveals that the blood of newborns is more susceptible to Heinz body formation with that of prematures being most susceptible.