Thread: (Play?) fighting

Last night, Walter and one of my moggies (Jack) seemed to really get into it and I'm not certain whether it was just really aggressive play or more serious. Jack is a neutered male who has always been a bit of a bully to the other moggies, but not -- as far as I've seen -- to Walter. Walter is 8 months old today and still intact.

When I looked, both were lying on their sides and kicking each other with their rear feet. Both seemed to be trying to bite at each other and there was a lot of noise. It lasted several seconds and then Walter ran up the stairs with Jack chasing him. A few seconds later, Walter came running back down and went into the laundry room, Jack was right behind him, but did not follow him into the laundry room. Jack went a different direction which brought him face-to-face with another moggie who growled at him (which in itself isn't unusual).

So, how can I determine if this is serious? If serious, what can or should I do?