Summary.Carbon‐monoxide‐hemoglobin methods of determination that deal with strongly diluted blood must give too low values owing to the fact that carbon‐monoxide‐hemoglobin dissociates on dilution. A simple method, sufficiently accurate for clinical use, has been worked out from which these sources of error have been eliminated.The determination is easy to carry out and takes only a few minutes. An important advantage of the method is that it enables determination on a micro‐scale, so that blood may be taken from the tip of the finger. Samples can be sent in capillary tubes, as for instance is done in the case of blood‐tests for alcohol according to Widma