Certain photochemical and enzymic activities of isolated chloroplast preparations were evaluated in tobacco plants susceptible ( Nicotiana tabacum cv. Bright BC 60) and hypersensitive ( N. tabacum cv. Havana 425) to tobacco mosaic virus (TMV) infection. The main results were as follows: 1. (i) Chloroplasts isolated from inoculated (asymptomatic) and systemically infected (with mosaic symptoms) leaves of the susceptible plants did not exhibit any change in the rates of nicotinamide adenine dinucleotide phosphate (NADP) and ferricyanide photoreduction expressed as μmol mg −1 chlorophyll. In contrast, chloroplasts isolated from inoculated leaves of the hypersensitive plants, starting at the beginning of local lesion expression (collapsed areas, 2 days after inoculation; necrotic areas, 3 to 4 days after inoculation) exhibited a strong inhibition of electron transport reaching values about 60–70% of the control. This effect was observed when water was the electron donor, irrespective of whether NADP or ferricyanide was the electron acceptor; no differences were observed when NADP reduction occurred via Photosystem I after 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) inhibition, using ascorbate and 2,6-dichlorophenol indophenol (DCPIP) as an artificial electron donor. 2. (ii) The rate of oxygen photoreduction, expressed as μmol mg −1 chlorophyll, was slightly-increased in the chloroplasts from hypersensitive leaves 1 day after inoculation and then reduced to values about 40% of the control 2–4 days after inoculation. Less oxygen photoreduction was also observed in chloroplasts from systemically infected leaves, while no variation was observed in the chloroplasts from inoculated leaves of the susceptible plants. 3. (iii) Ribulose-1,5-bisphosphate carboxylase activity (measured in chloroplast extracts) was decreased in inoculated hypersensitive leaves during differentiation of local lesions (2 to 4 days after inoculation), reaching values of 30–40% of the control. A less intense decrease of this activity was observed in uninoculated leaves of the susceptible plants displaying mosaic symptoms which did not exhibit any significant variation in photochemical activity (NADP and ferricyanide photoreduction). 4. (iv) Other enzymatic activities measured in chloroplast extracts and/or total-leaf extracts, including malate dehydrogenase nicotinamide adenine dinucleotide-dependent (NADH-dependent) glutamate oxalacetate transaminase, superoxide dismutase, and phosphoenolpyruvate carboxylase, were either unchanged or little affected by TMV infection both in susceptible and hypersensitive combinations. These results are discussed with respect to the hypothesis that the hypofunctionality of the photosynthetic apparatus, characteristic of chloroplasts from hypersensitive leaves, may contribute, in part, to the localization of infection and thence to host resistance.