Mammoth Uncensored Word List:
13 words that are 21 letters long beginning with i in the mammoth uncensored word list.

immunocytochemistries immunoelectrophoreses immunoelectrophoresis immunoelectrophoretic incomprehensibilities indistinguishableness institutionalizations interchangeablenesses interconvertibilities internationalizations intersubstitutability isopropylcyclohexanes isopropylcyclohexenes