Numerous studies have examined kinetics of T-cell functional responses following non-specific and antigen-specific stimulation in vitro. However, while reports of phenotypic T-cell changes after non-specific stimulation are abundant, only little information on phenotypic effects of antigen-specific stimulation is available. In the present study we have examined phenotypic T-cell changes after in vitro stimulation by the antigens purified derivative of tuberculin (PPD) and tetanus toxoid (TT). We show that the well-established differences in kinetics of mitogen- and antigen-induced T-cell proliferation in vitro is paralleled by differential kinetics in the expression of the T-cell adhesion and activation antigens leukocyte function-associated antigen 1 (LFA-1; CD11a/CD18), interleukin-2 receptor (IL-2R; CD25), and intercellular adhesion molecule 1 (ICAM-1; CD54). Furthermore, the changes in expression of all 3 surface antigens showed similar kinetics, and correlated with the magnitude of the lymphoproliferative response. By day 8 (PHA-stimulation) or day 12 (PPD or TT stimulation), the lymphoproliferative response was essentially completed, the expression of CD11a and CD54 had approached prestimulation levels, and CD25 expression was decreasing. This indicates that T-cell expression of all the 3 surface antigens examined is reversible. While this is in agreement with previous reports of the expression kinetics of IL-2R and ICAM-1, this is the first report indicating that the regulation of T-cell surface expression of LFA-1 is bidirectional. The results are discussed in relation to phenotypic characterization of memory T cells.