The effect of Tamm Horsfall protein (THP) of 18 healthy subjects and 14 diabetics on adherence of Escherichia coli (06:K13) 2699 strain to human kidney cells (HUK) was studied. Adhesion of bacteria (without additions: 100 bacteria per cell) was reduced dose-dependently by THP, half maximal inhibition occurring with 250 micrograms THP ml-1. Maximal inhibition (-84% at 1000 micrograms ml-1) exceeded inhibition by alpha-methyl-mannoside (36% at 50 mM), was specific (not reproduced by other glycoproteins, e.g. ovalbumin, mucin or thyroglobulin) and reversible (abolished by washing THP off HUK cells). Anti-adherence property of THP was not abolished by neuraminidase treatment. No significant difference of anti-adherence activity of THP was found between controls and diabetics, despite altered carbohydrate composition of THP in diabetes.