To overcome the disadvantages of the presently available hemoglobinometers, we have developed an optical instrument that measures the total hemoglobin (Hb) concentration in whole, undiluted blood. The device uses an infrared light-emitting diode to illuminate a capillary tube filled with a sample of whole blood. Light scattered in the blood travels a short distance down the length of the capillary tube, passes through a second light path, and reaches a photodetector, the output of which is amplified, digitized, and fed into a microprocessor. The microprocessor computes the Hb concentration as a nonlinear function of the light intensity. The optical device yielded Hb content measurements that correlated well with standard methods (r = 0.99, slope = 0.94, mean absolute difference = 0.75 g Hb/dl). Thus the accuracy appears to be less than 1 g Hb/dl. The advantages of the present device are as follows: 1) no chemical reaction is required (hence neither accurate dilutions nor toxic reagents are necessary); 2) it reads Hb concentration within a few seconds; 3) it can be operated by unskilled personnel; 4) it could be made portable and thus could be operated in the field, in rural settings, or at accident sites; 5) sample size is small (25-70 microliters); and 6) the same capillary tube can be centrifuged if a measure of hematocrit is also desired. A detailed parts list and circuit diagram are presented, and sources of error are discussed.