By freeze-drying, the infectivity of crude extracts of plants infected with tomato spotted wilt virus was reduced to 4% of the original infectivity, whereas the preparations containing 1% cysteine maintained 81% of their original infectivity. Freeze-dried preparations of crude extracts or plant tissues lost their infectivity after preservation for 4 months at 4C, while the preparations containing 1% cysteine remained infectious for 24 months.