Dilantin (sodium 5,5'-diphenyl-hydantoinate) induces hyperplastic changes involving increased collagen, in the gingivae. Our experiments indicate that Dilantin inhibits protocollagen proline hydroxylase (PPH) in a noncompetitive manner and the inhibition is reversed by adding more Fe2+. Our observations suggest that Dilantin may inhibit PPH by complexing Fe2+ and it may not be directly involved in increased collagen synthesis.