The purpose of this work was to optimize the growth conditions for scale-up of mass production of the main phospholipid (MPL) fraction from the archaebacterium Thermoplasma acidophilum. T. acidophilum was cultivated in flasks in the presence of P-32-ortho-phosphate under varying conditions. The lipids were extracted from the lyophilized cells and analysed by means of two-dimensional thin-layer chromatography. Autoradiography detected up to 11 different fractions. The monoglycosyl derivative of the MPL amounts to 60-80% and thus constitutes the main fraction of the total phospholipid. Variations ingrowth temperature, pH, yeast extract (YE), glucose and substitution of glucose by citrate were tested. The cells were examined by conventional and electron microscopy. During growth, the pO(2) decreased considerably, indicative of rapid O-2 consumption by the Thermoplasma cells. Growth under low O-2 tension, without addition of O-2, and under gassing with N-2 or CO2 revealed that T. acidophilum is obligatorily aerobic. For large cell mass production the addition of YE was varied quantitatively and at time intervals. The apparently most economic procedure in the fermentors is : growth at 59 degrees C and pH 2.0, with 10 g glucose/l and total amount of YE 6 g/l, applied in three portions, 2 g/l each, initially, after 18 and 24 h. Harvesting after 40 h yielded 0.66 g/l of cell dry mass. Scaling-up was successful from 1-l flasks to 10-l and 50-l fermentors under aeration of 0.02-0.04 vvm.