Determine the original medium to which the enclosure was attached. Ask whether the source originally appeared with a written letter or if it was an e-mail attachment.
Follow the citation guideline of your discipline for citing the medium to which the enclosure was attached. For example, the Modern Language Association, or MLA, requires authors to cite letters following this format: Last name, first name. Descriptive note on the letter (such as "Letter to George Washington"). Date the letter was sent. Collection in which the letter is stored if available. Format (such as print or e-mail).
Insert the word "Enclosure" between the descriptive note on the letter and the date on which the letter was sent for MLA format. For example, if you cite a resume that was enclosed with a letter that Mr. A. P. Pliccant sent to Ms. H. R. your citation would look like "Pliccant, A. P. Letter to H. R. Enclosure. 25 Jan. 2009. City of Norfolk, VA Open Records. Print."
Include the initial of the first name, the last name and the date the item was sent for Americal Psychological Association format. For example, cite the same letter as above as "A. Pliccant (personal communication, January 25, 2009)" in APA format.