An antimicrobial agent is produced by the cyanobacterium Synechococcus leopoliensis which was found to be active against the Gram-positive bacterium Staphylococcus aureus. The effects of temperature, pH, incubation period, some media and different nitrogen and carbon sources on both growth and antimicrobial activity were investigated. Temperature 35 degrees C and pH 8 were the best for growth and antimicrobial agent production and 14 and 15 days of incubation were found to be the best for maximum growth and antimicrobial activity, respectively, in the medium BG-11. No antimicrobial activity could be detected by the use of G medium, moderate activity was recorded with Chu 10 medium, while high activity was reported in BG-11 medium. Leucine was the best nitrogen source for antimicrobial activity, while maximum antimicrobial activity was introduced by using the carbon sources, citrate and acetate. Very high antimicrobial activity could be detected by using the carbon source galactose in combination with the nitrogen source alanine or by using arabinose with methionine.