This study investigated the effects of ponesimod, a selective S1P 1 receptor modulator, on T lymphocyte subsets in 16 healthy subjects. Lymphocyte subset proportions and absolute numbers were determined at baseline and on Day 10, after once-daily administration of ponesimod (10 mg, 20 mg, and 40 mg each consecutively for 3 days) or placebo (ratio 3:1). The overall change from baseline in lymphocyte count was −1,292±340×10 6 cells/L and 275±486×10 6 cells/L in ponesimod- and placebo-treated subjects, respectively. This included a decrease in both T and B lymphocytes following ponesimod treatment. A decrease in naïve CD4 + T cells (CD45RA +CCR7 +) from baseline was observed only after ponesimod treatment (−113±98×10 6 cells/L, placebo: 0±18×10 6 cells/L). The number of T-cytotoxic (CD3 +CD8 +) and T-helper (CD3 +CD4 +) cells was significantly altered following ponesimod treatment compared with placebo. Furthermore, ponesimod treatment resulted in marked decreases in CD4 + T-central memory (CD45RA −CCR7 +) cells (−437±164×10 6 cells/L) and CD4 + T-effector memory (CD45RA −CCR7 −) cells (−131±57×10 6 cells/L). In addition, ponesimod treatment led to a decrease of −228±90×10 6 cells/L of gut-homing T cells (CLA −integrin β7 +). In contrast, when compared with placebo, CD8 + T-effector memory and natural killer (NK) cells were not significantly reduced following multiple-dose administration of ponesimod. In summary, ponesimod treatment led to a marked reduction in overall T and B cells. Further investigations revealed that the number of CD4 + cells was dramatically reduced, whereas CD8 + and NK cells were less affected, allowing the body to preserve critical viral-clearing functions.