The cytoplasm of tumor cells from a subdermal nodule in a patient with fibrodysplasia ossificans progressiva (FOP) exhibited intense enzymatic activity in cryostat sections processed for demonstration of alkaline phosphatase. Nuclear heterochromatin and nucleoli, particularly in the area of the dense component, also showed strong reactivity. Finely minced blocks from the lesion of the patient with FOP revealed lighter reactivity which, in the tumor cells, avoided membrane limited spaces and appeared to be confined to hyaloplasm. Extracellular spaces disclosed very little or no reactivity and specimens from the patient's uninvolved skin lacked staining. The tumor cells from the subdermal nodule did not exhibit increased acid phosphatase activity. Cells (L-FOP) derived from a subdermal nodule and grown by tissue culture techniques also synthesized large amounts of prostaglandin E-like material and alkaline phosphatase. The amounts of prostaglandin E-like material synthesized by these L-FOP cells was reduced by more than 31 per cent by the antiinflammatory drugs indomethacin and sodium meclofenmate. Also, the production of alkaline phosphatase by these L-FOP cells was reduced by more than 40 per cent by ethane-1-hydroxyl-1,1-diphosphonate. Addition of prostaglandin E to nonlesion cells did not result in increased alkaline phosphatase activity.