$1.96{ imes}10^{-5}$(IU/cell/hr) of specific scu-PA production rate was obtained from HEK cells in maintaining ca. $8{ imes}10^{5}$(cells/ml) of maximum roll density at 10(ml/hr) of perfusion rate with recycling 20% serum free conditioned media. It can be compared to $4{ imes}10^{6}$(cells/ml) of maximum cell density and $4.56{ imes}10^{-4}$(IU/cell/hr) of specific production rate in cultivating cells with 1% serum containing medium. Thc conversion ratio of scu-PA to tc-UK increased up to 55% as the recycling ratio increased; however, recycling the used medium seemed to have least negative effect on cell growth. It also showed that the recycling process had definitive advantage of using serum free medium in perfusion cultivation of HEK cell line.