The kinetics of circulating antigen sensitive T-cells were studied in Hartley strain guinea pig recipients of Shorthair strain first- and second-set skin allografts. Peripheral blood donor antigen sensitive T-cells (AST) were quantitated by the antigen-stimulated active rosette-forming T-cell (AgARFC) assay by incubating lymphocytes in the presence and in the absence of soluble transplantation antigens. The number of circulating AST/cu mm rose to maximum levels (1,165 +/- 272 SEM) by day 3 and fell sharply before first-set graft rejection (453 +/- 117 SEM) on day 7 after transplant. In contrast, there were no detectable antigen-sensitive cells when lymphocytes from both recipient and control guinea pigs were stimulated with soluble recipient-strain antigen. Significant numbers (212 +/- 159 SEM) of circulating AST remained through day 68 after first-set grafts. Following placement of sencon-set allografts on day 73, the AST disappeared from the circulation for 2.5 days and then rose to peak levels (825 +/- 167 SEM) in circulating AST (579 +/- 327 SEM) preceded rejection of second-set skin allografts. When control guinea pigs were immunized with a single dose of soluble donor antigens, a progressive increase in circulating AST (579 +/- 327 SEM) was found through day 17 after sensitization without the fall associated with graft rejection. The antigen-stimulated, rosette-forming T-cell assay may prove useful in the detection of cellular presensitization and in the monitoring of graft rejection in clinical transplantation.