PAP-1 (5-(4-phenoxybutoxy)psoralen) is the selective inhibitor of Kv1.3, voltage-gated K+ channel, that is highly expressed in cell membranes of activated effector memory T cells (TEM). The blockade of Kv1.3 results in membrane depolarization and inhibition of TEM proliferation and function. PAP-1 is 23-fold selective over Kv1.5, 33- to 125-fold selective over other Kv1-family channels, and 500- to 7500-fold selective over Kv2.1, Kv3.1, Kv3.2, Kv4.2, HERG, calcium-activated K+ channels, Na+, Ca2+, and Cl- channels. PAP-1 does not exhibit cytotoxic or phototoxic effects, is negative in the Ames test, and affects cytochrome P450-dependent enzymes only at micromolar concentrations. PAP-1 potently inhibits the proliferation of human TEM cells and suppresses delayed-type hypersensitivity, a CD4+ T cell-mediated reaction, in rats when administered intraperitoneally or orally. PAP-1 further effectively treats allergic contact dermatitis, a CD8+ T cell-mediated reaction.