The influence of enhanced O-2 concentration on growth and formation of secondary metabolites by Streptomyces griseoflavus (strain Tu 2880) was investigated in a stirred tank and in an air-lift fermentor. At a partial pressure of O-2 P-02 = 1880 mbar the growth was lowered by 50% compared to p(o2)=2l0mbar, whilst substrate consumption and 02 uptake rate increased markedly. Production of the colabomycin complex reached maximum values at P-o2 =630 mbar. A similar increase of secondary metabolite formation was obtained when glycerol or acetate were fed at P-o2 =220 mbar. The portion of the derivate colabomycin A in the product mixture rose from 43% at p(o2)=210 mbar to 73% at p(o2)=1260 mbar. Since dissolved 02 concentration has a significant influence on productivity and selectivity it may be used to regulate aerobic fermentation processes.