RP 001 is a potent sphingosine-1-phosphate receptor 1 (S1P₁) agonist (EC₅₀ = 9 pM). It is competitive with W146 , a selective S1P₁ antagonist, and induces internalization and polyubiquitination of S1P₁ in a dose-dependent manner. RP 001 induces dose-dependent lymphopenia in mice (EC₅₀ = 0.03 mg/kg). It reduces S1P₁-EGFP expression ex vivo on lymphocytes in the isolated lymph node of Edg1^eGFP/eGFP mice but has no effect in vivo. RP 001 also completely inhibits M. stadtmanae (MSS) rechallenge-induced enlargement of B cell-rich tertiary lymphoid tissues (TLTs) and reduces neutrophil accumulation in the airways of a mouse model of experimental hypersensitivity pneumonitis.