Control and TMV-infected leaf tissues of three varieties of tomatoes, Marglobe, Moscow and Delicious, were analyzed for Ribulose Diphosphate Carboxylase (RuDPCase). The specific activity of RuDPCase based on protein concentration determined by peak area obtained with the Model E Ultracentrifuge, specific activity of RuDPCase based on protein concentration determined by Lowry's method, total amount of RuDPCase per gram fresh weight, total protein of plant leaves, total chlorophyll, and ratio of Chlorophyll a to Chlorophyll b were determined. Tobacco Mosaic Virus (TMV) infection resulted in an increase in specific activity of RuDPCase and the amount of total protein (at 2 days after inoculation). During the same time period there was a decrease in the total amount and total activity of RuDPCase in TMV-infected plants. The TMV infection appeared to also impair the synthesis of chlorophylls. The abnormal growth of TMV-infected plants appeared to be associated with the utilization of energy and organic substances from host plants for TMV-particle multiplication. As shown by changes in RuDPCase, photosynthesis in TMV-infected plants was disturbed.