John Stuart Mill, Utilitarianism, in Frasier's Magazine For Town and Country, Vol. LXIV, October−December (London: Parker, Son, and Bourn, West Strand, 1861). Chapter I and II, p. 391-406 (October); Chapter III and IV, p. 525-534 (November); Chapter V, p. 659-673 (December). This is the very first publication of Utilitarianism. It was published separately as a book in 1863 by the same publishers (see here).