The effects of prostaglandin E(1) (PGE(1)) and prostaglandin F(2α) (PGF(2α)) on bone formation were investigated with femur and tibia from 10-day chick embryo. When the bones were cultured in serum free medium, PGE(1) (10nM) caused a significant increase of bone alkaline phosphatase activity and a slight increase in bone protein content, whereas PGF(2α) (100nM) significantly decreased the enzyme activity. On the other hand, in a medium containing 20% serum, PGE1 did not affect the enzyme activity at 10nM, but slightly increased the activity at 1nM. No effect of PGE(1) on bone calcium content was observed in a concentration ranging from 0.1nM to 100nM. Thus, PGE1 was likely to stimulate bone formation at 10nM, whereas PGF(2α) supressed formation at 100nM.