Clinical investigation with diabetics was made whether biological potency and duration of action of highly-purified monocomponent (MC) Lente insulin are different from conventional insulin or not. Diurnal variation in blood sugar of 30 insulin-dependent diabetics was measured once a week by cross-over method and statistical consideration was made on the mean blood sugar level of both groups. As a result, no statistically significant difference was recognized as to the mean blood sugar level either in diurnal variation of both MC Lente and conventional Lente (p>0.05) or in M-value, a blood sugar control indicator advanced by Schlichtkrull and modified by Goto et al. The highest correlation of blood sugar level with M-value was seen at the time of peak blood sugar in a day (r=0.937), while that with fasting blood sugar level seen both at noon (r=0.864), at 16: 00 (r=0.795) and at 7: 00 (r=0.491) in said order. The result suggests that assessing diabetic control only with morning fasting blood sugar level may not always be dependable.