Synopsis Monolayer cultures of human fibroblasts were used both to confirm clinical experiments in which it was found that placental extracts increased the rate of healing and to demonstrate the action on cellular regeneration. The cytotoxic dose of lyophilized extracts from human placentae was first determined and the extracts were then added to culture media at concentrations of 100 to 400 mug/ml. A variable amount of newborn calf serum was used in order to obtain media lacking nutrient substances. The tests were carried out on cells from both young and elderly donors and growth was evaluated by counting cell numbers and by microassay of DNA. A stimulatory action of the placental extracts was only observed on young cells cultivated in deficient culture media and no stimulation could be demonstrated on cells aged in vivo or in vitro. Effets des extraits placentaires sur la croissance des fibroblastes in vitro.