The phosphorodiamidate Morpholino oligomers (PMO) are a new class of antisense agents that inhibit gene expression by binding to RNA and sterically blocking processing or translation. In a search for a Morpholino agent that would inhibit cell proliferation, it was found that oligomers directed against c-myc, a gene involved in control of the cell cycle, were effective. The sequence specificity and mechanism of action of one agent were determined. The 20-mer 126 lowers c-myc protein levels in treated cells and arrests cells in G0/G1 of the cell cycle. It also acts at the RNA level to inhibit normal pre-mRNA splicing and instead produces an aberrantly spliced mRNA. Irrelevant and mispair control oligomers indicated that the observed antiproliferative effect was sequence specific. This was confirmed in a reporter gene model system using a c-myc 5'-untranslated region (5'-UTR) fused to a cDNA copy of the insect luciferase gene. We conclude that 126 is acting through an antisense mechanism involving Watson-Crick hydrogen bonding to its target RNA. A specific antisense agent directed against a cell cycle-associated gene mRNA may be useful as a therapeutic in diseases characterized by excess cell proliferation, such as restenosis following balloon angioplasty or cancer.