Effect of low concentration negative air ions on ,Streptomyces avermitilis, on solid culture medium was studied. The microorganism was treated consecutively with negative air ions of 5×10,3,/cm,3, in 10 days. It was found that under the treatment,Streptomyces avermitilis, reached the plat-stage production 4 days earlier than the control. Moreover, quantities of both effective contents of B1a and B1b increased by about 20%, while no remarkable changes in the ineffective contents were observed. Repeated experiments prove that the negative ion treatment is a useful way to shorten long fermentation cycle and to increase antibiotic production.