Police officers were called to the flat in Stafford Road in Brighton yesterday at approximately 02:00 after a report of a disturbance.
When they forced an entry into the flat they found an unidentified man in his 40s with serious chest and head injuries. At the same time two men escaped from the rear of the building and ran across gardens behind the road. Both were detained in Exeter Street shortly after 02:30.
The injured man was pronounced dead at the Royal Sussex County Hospital at approximately 04:00.
A 44-year-old man from Brighton and a 45-year-old man of no fixed address were arrested on suspicion of attempted murder and were still in custody on Wednesday morning.
The police are appealing for anyone with information about the deceased or the incident to report online or call 101 quoting Operation Barnfield.