The effect of insulin on the incidence of experimental Serratia marcescens cystitis in alloxan-induced diabetic mice was studied. The symptoms in diabetes were improved by injection of insulin (1 I.U./b.i.d.) for 3 days. Diabetic mice treated with insulin showed lower susceptibility to cystitis with S. marcescens than non-treated diabetic mice, but slightly higher than normal mice. It was, therefore, suggested that the insulin treatment was important for prevention of S. marcescens cystitis. The prevention of cystitis in insulin-treated diabetic mice was possibly due to the increase of urea nitrogen that inhibits the growth of S. marcescens in urine.