We have studied the stability of 14 serum and blood constituents after 4 days transport by mail at ambient temperature. For statistical evaluation a graphical truncated normal sequential test was used. A constituent was considered stable when the change of the mean concentration of the constituent was less than an amount equal to one standard deviation of the analytical method used, with 5% risk of error for the decision. Under these conditions 9 constituents were stable: serum albumin, creatinine, sodium, potassium, calcium, alkaline phosphatase, gamma-glutamyltransferase, blood haemoglobin and blood erythrocytes. Instability was found for serum bilirubin, blood leucocytes, haematocrit and erythrocyte volume related variables. The experimental design allows stability data to be collected during field testing by using daily runs in the laboratory. Data of this kind are essential for making a reliable laboratory service available to primary care units. The demands of the clinical situation should, however, be taken into consideration.