L 929 mouse fibroblasts were cultured as monolayer using MEM Eagle medium after addition of bovine serum albumin, antibiotics, antimycotica, ferrous nitrate and ascorbic acid. In 24 hours distances cell number was counted. At the same time peptidyl prolyl hydroxylase activity was determined and related to the protein concentration in the cells. The proliferation of the cells followed a typical curve. The cell number decreased after trypsination, then increased logarithmically, finally reached a stationary phase. Enzyme activity showed a correspondent curve. Active enzyme did not appear until near the end of the log phase. After addition of the mesenchyme active substances in therapeutical concentrations proliferation and synthesis of active peptidyl prolyl hydroxylase in the cells were significantly decreased. The results are discussed in relation to the characteristics of collagen synthesis and proliferation of connective tissue cells under physiological, pathophysiological and therapeutical conditions.