In this study, the cells producing T-cell growth factor (TCGF) in the rat MLR were characterised with respect to the antigens defined by W3 13, W3 25, and OX8 monoclonal antibodies. Unfractionated lymphocytes and cells depleted of OX8 positive cells were found to be fully capable of producing TCGF, whereas lymphocytes depleted of W3 13 and W3 25 positive cells had lost this ability. Parallel experiments demonstrated that cells selected by the fluorescence-activated cell sorter for the expression of W3 13 and W3 25 defined antigens were potent producers of TCGF. Further studies suggested a functional role for the antigen defined by W3 25 antibody because the addition of this antibody to a MLR abrogates TCGF production. These findings suggest that the important immunomodulatory functions of W3 25 positive lymphocytes could be exercised via the synthesis of essential lymphokines.