A subroutine for evaluating the acid-base status in human blood is described. It employs as inputs seven parameters: pH-pO2-pCO2-Hb-Ht-the subject's temperature T and the plasmaprotein content PP and is based on a series of mathematical interrelationships existing between these various blood constituents. As output it returns the base excess (BE) of the whole blood and a number of pertinent data relating to the acid-base status and to the gas transport mechanisms in erythrocytes and plasma. An important advantage of this subroutine on other major procedures is that it can be accommodated into the memory space of a medium seized electronic calculator.