Mammoth Uncensored Word List:
35 - 13 letter words starting with is

islamophobics ismaticalness isoagglutinin isoalloxazine isoantibodies isobarometric isocarboxazid isocheimenals isochromosome isochronising isochronizing isochronously isodimorphism isodimorphous isoelectronic isogeothermal isogeothermic isolabilities isolationisms isolationists isomerisation isomerization isometrically isoperimeters isoperimetric isoprenalines isoproterenol isosmotically isospondylous isostatically isosthenurias isoteniscopes isotonicities isotretinoins isotropically