Growth conditions were investigated in batch culture of the thermotolerant cyanobacterium Phormidium sp. with the aim of identifying which culture conditions modulated an increase in the yield of bioactive material. This was achieved using a factorial experimental design, followed by a method of steepest ascent. Although temperature, nitrilotriacetic acid, nitrate and ferric iron chloride all influenced regulation of the production of compounds which inhibit the growth of Staphylococcus aureus and Candida albicans, it was found that ferric iron chloride exerted the greatest effect.