Abstract: An aqueous solution for prophylactic treatment of teats of lactating mammals, comprising as a first component at least partially deacetylated chitosan, or an acid addition salt thereof, in a concentration of up to about 2% by weight of chitosan, the solution having a pH of from about 4 to about 6.8, and said first component having a molecular weight such that the viscosity of the solution is less than about 50 mPas; and a method of prophylactic treatment of teats of lactating mammals.