Culture filtrates (conditioned medium, CM) containing exudates obtained from cells of Scenedesmus subspicatus grown in batch culture were tested for their autoinduction activity. Undiluted CM completely inhibited the proliferation of cells due to depletion of nitrogen in this medium. When undiluted CM was supplemented with fresh bold basal medium (BBM) medium, enhancement of population growth in a dilution-dependent manner was observed. The most effective was 2-fold diluted CM (CM/2), whereas the growth activity of CM/20 decreased considerably. The proliferation of cells at a very low initial density (10 cells mL-1) was induced by the addition of CM/2. Biovolume, dry matter production and oxygen evolution by cells grown in 2-, 5- and 10-fold diluted CM increased markedly in both a concentration- and time-dependent manner. Conditioned medium factor (CMF) was mainly a product of rapidly dividing cells. Preliminary characterisation revealed that CMF is heat-stable, resistant to very low pH, highly hydrophilic (poorly soluble in ethanol or diethyl ether) and diffuses through a 1 kDa dialysis membrane. CM can be stored for long periods due to the time-stable activity of CMF. Depending on the enzyme applied, proteolytic digestion completely (papain, bromelain) or partially (subtilisin) abolished the activity of CM. This effect of CM was enhanced by digestion with trypsin. We suggest that CMF may be a low-molecular-weight peptide(s) or glycopeptide(s).