Production of pH-sensitive pigment (actinorhodin-like substance) were ascertained from ten Streptomyces isolates. Streptomyces IQ45 isolate was the only isolate which produced pH-sensitive pigment. The production of pH-sensitive pigment was detected by fuming over ammonia. After extraction of this antibiotic a number of physicochemical examination were carried out which involved (IR, UV, MP, CHN-analysis and solubility test) indicated that this antibiotic is actinorhodin-like substance. TLC of extracted substance showed single spot with R/ value equivalent to (0.26) which was close to that of actinorhodin. This antibiotic showed inhibitory activity against Staphylococcus aureus similar to that of actinorhodin produced by Streptomyces coelicolor A3(2). Productivity of this antibiotic was (45 mg/L) at pH 8.5 and (40 mg/L) at pH 7 from mycelia! mat and (10 mg/L) when extracted from liquid medium at pH 8.5, while no actinorhodin-like substance from liquid medium at pH 7. White colony mutants (defective in actinorhodin productio.n) were isolated after mutagenesis using u.v. light and the u.v. doses measured using Ferrioxalate actinometric method that the mutants were isolated after u.v. mutagenesis at a dose equivalent to 361.2 ergs/mm2/sec. Grey wild type and white mutants were characterized morphologically and physiologically. Both showed that they were identical except in colour of aerial mycelia (CAM), sporophore shape and actinorhodin-like substance production. Genetic crosses between the grey wild type and white mutants indicated that actinorhodin encoding gene located between str & rif I genes on the chromosome. Plasmid curing by ethidium bromide indicated that the plasmid of DNA of IQ45 isolate has no role in actinorhodin-like substance production . Two plasmid bands were detected in the original isolate, these could not be detected in the cured colonies.