DL-threo dihydrosphingosine is a cell-permeable inhibitor of the extracellular signal-regulated kinase (ERK) signalling cassette in mammalian cells. DL-threo dihydrosphingosine is effective against both growth factor- and G-protein-dependent activation of ERK. DL-threo dihydrosphingosine may be usefully employed to block smooth muscle cell proliferation.