This paper reports the early energetic response of plant cells to infection by pathogenic mycoplasma. The microalgal cells of Chlorella vulgaris were used as a plant model system to investigate the stress responses against the Mollicute (mycoplasma) Acholeplasma laidlawii. The chlorella cells were cultured under various physiological conditions and the dependence of the defence responses to the action of the different concentrations of pathogen were studied in detail. The mycoplasma triggered a substantial increase in the rate of heat production and caused an enhanced oxygen uptake rate, which were regarded to be manifestations mostly of the production of the superoxide anion radical as one of the active oxygen species (AOS). Differences were observed in the intensity and duration of the "heat burst" and the "oxygen burst" under the different conditions of the experiments and were interpreted in terms of the oxygen metabolism to form AOS, particularly, the superoxide anion radical, that are important in the host defence. Control chlorella cells gave a calorimetric-respirometric (CR) ratio of -509 +/- 73 (S.E.) kJ/mol O-2, within the calculated range for the oxycaloric equivalent (-509 (+/-15%) kJ/mol O-2) and meaning that the catabolic process was fully oxidative. At least part of the reason for the differences between the two "bursts" to give a CR ratio of -575 kJ/mol O-2 at 2.5 x 10(6) mycoplasma cells/ml chlorella culture (5.3 x 10(8) cells/ml) is the fact that the mycoplasma are facultative anaerobes. There is evidence that a humoral factor produced by the mycoplasma elicits an early response by the host when added as a cell-free supernatant. Besides the immediate, primary response to the interaction seen in energetic terms, electron microscopic evidence is presented to show that the mycoplasma attached to the host cells and probably served to prolong the host defence response.