Mammoth Uncensored Word List:
35 - 12 letter words starting with is

islamophobes islamophobia islamophobic isoantigenic isobilateral isobutylenes isocheimenal isochromatic isochronally isochronised isochronises isochronisms isochronized isochronizes isodiametric isodiapheres isodimorphic isoenzymatic isogeotherms isolationism isolationist isomagnetics isometropias isomorphisms isoperimeter isoperimetry isoprenaline isostemonous isosthenuria isoteniscope isothermally isotonically isotopically isotretinoin isoxsuprines