A modification of the standard thermotherapy used to eliminate virus from potato virus X (PVX)-infectedSolanum tuberosum microplants of the Mexican National Potato Program is described here. Microplants were cultured with or without 10-5 M salicylic acid (SA) for 4 wk, then subcultured without SA and exposed to 42 C for 30 days. Survival was more consistent in SA-treated microplants: among seven genotypes, 40% to 100% of SA-treated microplants and 0% to 96% of controls survived at the end of thermotherapy. SA also improved survival of the post-thermotherapy subculture. Among surviving microplants, SA increased the virus-free yield to 100% from 40% to 65% in the controls. In an additional 30 PVX(+) genotypes, 98% of surviving SA-treated microplants were virus-free after thermotherapy, compared to 75% of controls. SA-treated microplants had lower catalase activity and higher hydrogen peroxide (H2O2) levels.