Mycoplasma contamination of cell culture is a serious problem in biomedical reseach. Three common PCR primers(F1,F2 and R1) were designed to amplify the spacer region between 16s and 23s DNA in rRNA operons of 6 species of mycoplasmas (M.arginini,M.orale,M. hominis ,M .hyorhinis,M fementans and A.laidlawii). When the DNA of 6 species was used as the template, primers F1 and R1 produced fragments of 340 to 468 bp,and primers F2 and R1 produced fragments of 145 to 211 bp. No discrete band was observed in electrophoretic gels when Hela cell or E.coli DNA was served as the template with the use of primers F1 and R1. As little as 8.5fg DNA of M.arginini，approximately 1 3 organisms could be detected. This suggests that mycoplasma contamination to cell cultures can be detected by PCR.