Oligo(nucleoside methylphosphonate)s complementary to the splice junction of herpes simplex virus type 1 immediate early pre-mRNAs 4 and 5 caused specific inhibition of herpes simplex virus type 1 growth. The dodecamer d(TpTCCTCCTGCGG) (deoxynucleoside methylphosphonate residues in italic) caused 50% and 98% decreases in herpes simplex virus type 1 titers at concentrations of 15 microM and 100 microM, respectively. d(TpTCCTCCTGCGG) inhibited viral but not cellular protein synthesis and decreased splicing of immediate early pre-mRNAs 4 and 5. Inhibition was highly sequence specific. A psoralen derivative of d(TpTCCTCCTGCGG) that can covalently bind to complementary sequences after exposure to 365-nm irradiation caused 90-98% inhibition of virus growth in cells treated with oligomer (5 microM) and irradiated at 1-3 hr postinfection. The data suggest that oligo(nucleoside methylphosphonate)s of appropriate sequence and derivatization may be effective as antiviral agents.