Mammoth Uncensored Word List:
52 - 11 letter words starting with is

isabellines isallobaric isapostolic ischuretics isinglasses islamophobe isoaminiles isoantibody isoantigens isobutylene isobutyrate isocheimals isochronise isochronism isochronize isochronous isocyanates isocyanides isodiaphere isodynamics isoelectric isoflavones isogeotherm isografting isoheptanes isoheptenes isoheptynes isolability isolecithal isoleucines isomagnetic isomerising isomerizing isometrical isometropia isomorphism isomorphous isoniazides isonitriles isopachytes isopentanes isopentenes isopentynes isopolities isopropanol isorhythmic isoseismals isoseismics isothermals isotonicity isotropisms isoxsuprine