Since ultralente insulin pharmacokinetics suggest faster absorption by human insulin when compared with bovine insulin using the subcutaneous route, the safety and efficacy of human ultralente in the outpatient setting was evaluated. Twenty type I patients participated in a randomized study using a crossover design of four six-week phases: (a) one daily injection of human ultralente; (b) two daily injections of human ultralente; (c) one daily injection of bovine ultralente and (d) two daily injections of bovine ultralente. Pre-meal human regular insulin was used with ultralente insulins and comprised 39 +/- 2% of the total daily insulin dose. Total and ultralente daily insulin doses were lower with human ultralente insulin (51.3 +/- 3.0 total and 30.9 +/- 3.4 ultra lente u/day) when compared to bovine ultralente insulin (57.8 +/- 4.4 and 36.1 +/- 4.4 u/day, p less than 0.01), yet the metabolic control achieved was virtually identical during both phases: (hemoglobin Alc 8.6 +/- 0.2% human vs. 8.4 +/- 0.4 bovine, p = NS). The frequency of mild hypoglycemia was 3.0 +/- 0.5 events per week vs 2.0 +/- 0.3 (p = NS). No severe hypoglycemia occurred. There were no differences between blood glucose daily profiles, insulin doses, hemoglobin Alc (8.6 +/- 0.4% BID vs. 8.4 +/- 0.3% QD injections) and occurrence of hypoglycemia between the single and two-dose long-acting regimens. These data indicate that long-acting semi-synthetic human insulin (a) can be effectively used as a once daily injection, (b) may be more biologically active than bovine, and (c) can be associated with safe and effective diabetes control.