To investigate the methodology to inhibit DNase activity in the cyanobacterium Spirulina, the effects of Mg2+ on the growth and DNase activity of Spirulina platensis strain SSP-14 were studied. The results indicated that strain SSP-14 could keep growing, but much slower, when inoculation and cultivation were done twice in Mg2+/free Zarrouk medium; and the activity of both intracellular and extracellular DNases was almost completely inhibited. This manipulation could efficiently remove the high-level activity of DNases which frequently exist in Spirulina and block foreign genes from being introduced. It is recommended that the technique be applied for genetic transformations of this alga.