The severity of acid-base disturbances in diarrhoeic calves was investigated and a simple, inexpensive method of monitoring them was evaluated. The Harleco apparatus measures the 'total carbon dioxide' in a blood sample, mostly generated from the bicarbonate present, and any abnormalities are mainly due to metabolic acidosis or alkalosis. Its performance was tested against a standard blood gas analyser by comparing the results obtained by both methods with nearly 2000 blood samples from healthy or diarrhoeic calves. After technical modifications, the technique gave excellent precision and accuracy for the clinical evaluation of acid-base balance, using venous whole blood. The samples were very stable, especially at 0 degrees C, but also at room temperature. The normal range (mean +/- 1.96 sd) for total carbon dioxide in whole blood from calves was 21 to 28 mmol/litre. For samples corresponding to mild, moderate or severe acidosis, 79 per cent were correctly classified by the Harleco apparatus and only 0.1 per cent were beyond the adjacent degree of severity. After four days of diarrhoea, the calves which later died had twice the deficit in plasma bicarbonate of those which survived. As death approached, the deficit was almost three times that in surviving calves and the blood pH shortly before death was as low as 6.79 +/- 0.08. The Harleco apparatus was less successful with alkalotic samples, but metabolic alkalosis is less common and usually less severe.