Two BHK cell lines (21 and 21/13) showed different rates of protein synthesis and degradation during growth in monolayer culture. During cell growth the rate of protein synthesis decreased progressively in BHK 21 cells, whereas in BHK 21/13 cells it remained constant. Furthermore, use of cell free systems showed that polysomes of BHK 21 were disaggregated markedly in the stationary phase, whereas those of BHK 21/13 were not. The rates of translation for protein synthesis in the two cell lines were similar, and were not affected by the growth state. The translation rates were measured as the times of ribosome transit and of peptide chain completion. This suggests that the decreased rate of protein synthesis in BHK 21 in the stationary phase may be due to a decreased rate of chain initiation.

The curves for decay of proteins labeled with ³H‐leucine in 30 minutes were identical in the two cell lines in the growing state, showing a biphasic pattern due to rapidly degraded components (T_1/2 = 16 h) and slowly degraded components (T_1/2 = 65 h). However, in the stationary phase the half‐life of the rapidly degraded components of BHK 21/13 became 5 hours, while that of BHK 21 remained 16 hours. The half‐lives of slowly degraded proteins in the two cell lines were similar. It was also shown by the method of approach to equilibrium that the proteins of BHK 21/13 cells were degraded about twice as fast as those of BHK 21 cells in the stationary phase.

Since there is no difference in the protein contents of the two cell lines at any stage of cell growth, it seems that during cell culture protein turnove of BHK 21 cells is controlled mainly by the rate of protein degradation.