A medium has been selected for providing the maximum synthesis (7-8 g/litre) of a free exocellular polysaccharide by Mycobacterium lacticolum 121. The following concentrations of the components are optimal for the biosynthesis: hexadecane, 5.0-6.5% (by volume); NaNO3, 2.0-4.0 g/litre; Na2HPO4-12H2O, 1.0-1.5 g/litre; KCl, 1.0-1.5 g/litre. An increase in the concentration of sodium nitrate and phosphate over 4.0 and 1.5 g/litre, respectively, favours growth of the cells but inhibits biosynthesis of the polysaccharide. Concentrations of hexadecane and potassium chloride over 6.5% and 1.5 g/litre, respectively, have no effect of the culture and yield of the polysaccharide.