Method and apparatus for determining the concentration of haemoglobin within red blood cells are provided. An undiluted, whole blood sample is deposited into a transparent analysis chamber (10) whose height is such that the red blood cells are in contact with both surfaces (14, 18). Then the optical density of a portion of the red blood cells in contact with both surfaces is measured. The haemoglobin concentration of the red blood cell is calculated from the optical density and the known distinction coefficient of haemoglobin. This is repeated with other cells until an average haemoglobin concentration can be determined with confidence. Dependent claims include use of zwitterionic isovolumetric sphering agent make the measurement independent of a geometrical form factor.