Phosphothioates may provide metabolic stability when compared to their phosphate counterparts, while retaining the potency and efficacy as agonists at sphingosine-1-phosphate (S1P) G-protein coupled receptors. Unlike their phosphate precursors, phosphothioate compounds with S1P-receptor profiles similar to that of FTY720, an emerging immunomodulator, were shown to evoke prolonged lymphopenia in vivo. Analysis of mouse plasma concentrations for a series of related alcohol/phosphate/phosphothioate compounds showed the conversion of the phosphate to alcohol. These preliminary data highlight the importance of metabolic regulation of S1P receptor ligands.