We searched for a new cell aggregation factor, and found one we called SAF in the mycelia of a strain of Streptomyces murinus. SAF was purified by active carbon and ion exchange column chromatographies and gel filtration of Sepharose 2B from the homogenized mycelia by sonication. SAF was stable from pH 7 to 9 at 37°C and up to 40°C at pH 8.0. The aggregation activity of SAF was maximum around pH 8.0 at 30°C, and the factor required for its activity metallic ions such as calcium and manganese. The aggregation activity of SAF was inhibited by laminarin, but it was not influenced by various other saccharides. SAF aggregated E. coli, S. aureus, M.luteus, sarcoma 180, and HeLa cells as well as S. marcescens, above all, its highest activity was toward B. subtilis, but P. vulgaris, P. aeruginosa, C. albicans, each type of human erythrocytes, and hepatoma 109A cells were quite resistant to SAF. These properties has proved that SAF is completely different from the other aggregation factors so far reported.