By February of 1944, the Red Army had started to field a new class of heavy tanks to replace the aging Churchills and KVs in the heavy tank regiments. They were given to Guards regiments, crewed by two officers and two senior sergeants each as befitted their perceived importance.
The tanks were built with a specific, but certainly not sole, criterion in mind: destroy Tigers. Testing against a captured example had shown that of all the weapons currently in Red Army service, the 52K 85 mm anti-aircraft gun seemed to be the most effective against that kind of tank, and the new heavy tank was designed around it; if a new tank were to enter service, it may as well be capable of defeating Tigers!
IS, or IS-85, tanks started production in September of 1943 and a grand total of 107 are estimated to have been produced (according to Baryatinksy).
Twenty-five of these tanks found their way into the 13th Guards Heavy Tank Shock Regiment, wherein a company of five ISs supported an attack on the town of Lisyanka. The timing was a little poor, and the five heavy tanks showed up at the tail end of an assault by T-34s of the 109th Tank Brigade. After having dealt with the T-34s, the defending German assets which included Panthers, PzKpfw IVs, anti-tank and assault guns, switched their attention to the ISs. Within ten minutes, two ISs were burning, and the other three driven back with multiple hits suffered per tank.
It was not the most auspicious debut for the new heavy tank, and the subsequent engagements over the next few weeks were not necessarily any better. Fortunately, IS-85 was really just a stopgap, with the substantially upgunned IS-2 already in production.