In cowpea leaves singly or doubly infected with cowpea chlorotic mottle virus (CCMV) and/or southern bean mosaic virus (SBMV), treatment with 2-thiouracil increased the accumulation of CCMV particles and strongly decreased the accumulation of SBMV particles. Thiouracil prevented the usual inhibition of synthesis of CCMV particles at about 6 days after inoculation, and 2–3 times as many CCMV particles accumulated as in water-treated plants. A single treatment of thiouracil 48 h before, at the time of, or 24 h after inoculation caused at least 90% decrease in the amount of SBMV particles extracted from inoculated leaves for at least 15 days. When virus particles were extracted from plants treated with thiouracil, the specific infectivity of CCMV was increased 2.3 times and SBMV was decreased 0.46 times as compared to virus from control plant