Two strains of diploid fibroblasts were derived from green monkey fetal lungs and their main properties were studied. These strains were shown not to differ principally from the well studied human diploid fibroblasts. The derived cultures are free from contaminating viruses. The titers of attenuated poliomyelitis virus strains achieved in green monkey diploid cells are comparable to those in primary cultures of green monkey kidneys. The possiblity of using green monkey diploid fibroblasts in virological practice is discussed.