The photosynthetic response to light can be accurately defined in terms of (1) the initial slope (quantum yield); (2) the asymptote (light-saturated rate); (3) the convexity (rate of bending); and (4) the intercept (dark respiration). The effects of photoinhibition [which damages the reaction centre of photosystem II (PSII)] on these four parameters were measured in optically thin cultures of green plant cells (Chlamydomonas reinhardtii). The convexity of the light-response curve decreased steadily from a value of 0.98 (indicating a sharply bending response) to zero (indicating Michaelis-Menten kinetics) in response to increasing photoinhibition. Photoinhibifion was quantified from the quantum yield of inhibited cells relative to that of control cells. The quantum yield was estimated by applying linear regression to low-light data or by fitting a non-rectangular hyperbola. Assuming the initial slope is linear allowed comparison with earlier work. However, as the convexity was lowered this assumption resulted in a significant underestimate of the true quantum yield. Thus, the apparent level of photoinhibition required for a zero convexity and the initial decrease in light-saturated photosynthesis depended upon how the quantum yield was estimated. If the initial slope of the light response was assumed to be linear the critical level of inhibition was 60%. If the linear assumption was not made, the critical level was 40%. At the level of inhibition where the convexity reached zero, the light-saturated rate of photosynthesis also began to decrease, indicating that this level of inhibition caused photosynthesis to be limited at all light intensities by the rate of PSII electron transport. At this level of inhibition the Fm-F i signal (where Fm is maximal chlorophyll fluorescence and Fi is intermedi-Abbreviations and symbols: Chl = chlorophyll content; DCMU = 3-(3,4-dichlorophenyl)-l, 1-dimethylurea; Fo, Fi, Fm= initial, intermediate, and maximal Chl fluorescence of dark adapted cells; P = rate of net photosynthesis per unit chlorophyll (gmol-(mg Chl)-1. s-1); PSII = photosystem II; PQ = plastoquinone ; q~ = initial slope to the light-response curve; 0=convexity (rate of bending) of the lightresponse curve of photosynthesis; Q=photosynthetically active photon flux density (400-700 nm, ~tmol. m-2. S-1) ate chlorophyll fluorescence of dark adapted cells; Briantais etal. 1988) from the fluorescence induction curve was zero and the FrFo signal (where Fo is initial chlorophyll fluorescence of dark adapted cells) was 30% of the control, indicating dramatic reduction or complete elimination of one type of PSII. These data do not contradict published mathematical models showing that the ratio of the maximum speed of electron transport in PSII relative to the maximum speed of plastoquinone electron transport can determine the convexity of the photosynthetic response to light.