In 36 unselected insulin-depending type I diabetics the reliability and practicability of the blood glucose self-control was investigated by means of the glucosignal test strip in the course of 12 weeks under everyday ambulatory conditions. In these cases the quality of the manual handling and of the visual evaluation of 896 test strips was judged in three series of application by special personnel in the laboratory by means of photometric evaluation. 7.5% of the test strips of all series were incorrectly handled and could not be evaluated photometrically. The correlation coefficients between the blood glucose concentrations which were visually valuated by the patient and measured photometrically in the laboratory were in the three investigation series 0.962, 0.964, and 0.950, i.e. the patients obtained a great reliability in the visual valuation of the test strips. Altogether 75-90% of all test strips were correctly judged by the patients. Deviations appeared in every case only by one concentration region, whereby there was an unequivocal tendency to underestimation, particularly in higher regions of blood glucose concentration. Prerequisite for a reliable handling of Glucosignal test strip by the patient is a thorough individual training.