The realization that soluble mediators of immune function, termed cytokines, can interact with nonimmunological cells has led to new insights into the role of the immune system in regulating cell growth and differentiation of other organ systems. The additional finding that dysregulation of these interactions can lead to a variety of clinical disorders has opened the possibility of new forms of therapy. A recent example of this aspect of cytokine biology comes from studies of a protein designated early T-lymphocyte activation-1/osteopontin (Eta-1/Op), which may play an essential role in immune resistance to certain bacterial and viral pathogens. Analysis of the structure of this protein has revealed that it is almost identical to several proteins associated with neoplasia and bone structure/function. The purpose of this review is to provide a summary of our current knowledge regarding the structure and function of Eta-1/Op and the potential relationship of this gene to several clinical disorders.