Disclosed are domain antibodies that monovalently bind CD28. Domain antibodies that are monovalent for binding of CD28 can inhibit CD28 activity. In one aspect, a domain antibody consists of or comprises a single immunoglobulin variable domain that specifically binds and antagonizes the activity of CD28, in an aspect, without substantially agonizing CD28 activity. In another aspect, the domain antibody is a human domain antibody. The disclosure further encompasses methods of antagonizing CD80 and/or CD86 interactions with CD28 in an individual and methods of treating diseases or disorders involving CD80 and/or CD86 interactions with CD28, the methods involving administering a domain antibody to the individual.