Radial-how bioreactors are effective for reducing concentration gradients, because the flow area is large and the path for circulating culture broth is short. In this study, a polyester nonwoven fabric mat was employed as a packing material in a radial-how reactor, and the renin production by recombinant Chinese hamster ovary (rCHO) cells in the bioreactor was investigated. The renin productivity of the cells was enhanced by use of an increasing circulating flow rate in the radial-bow reactor. It was also observed that the cells in the case of the increasing circulating how rate formed aggregates, and the specific renin production rate was 6 times higher than that in the case of a constant circulating flow rate. This may imply that the fell aggregation which occurred on the nonwoven fabric mat in the case of the increasing circulating how rate may have been due in part to the enhancement of the renin productivity.