Post-operative lung complications of varying severity are common. There are many factors involved. any of which may play a part in a given case. It is felt, however, that a common factor in all these cases is due to the action of ""H"" substance. The origin and action of ""Hï¿½ substance is indicated. Factors favourable to and antagonistic to the action of ""Hï¿½ substance are mentioned. A few case reports are added.