Fighting Cats

We have two cats. Lucy, who is spayed, is about 5 years old. She has been the only cat in the household for a long time and is not very affectionate or active. About 5 months age we brought home a male kitten, Cosmo. He in incredibly active and playful. When he was very small she did not like him at all, but he was curious about her. When he grew he also became more aggressive. He jumped on Lucy one day and bit her. The bite became infected and we took her docter and had it lanced. This cost about 250.00 and was very traumatic to Lucy. After the first attack we had Cosmo neutered at about 7 months. I thought that his agression would go away so I continued to let them stay together. Lucy does not run from Cosmo she only hisses and growls. Eventually he bit her again and we went back to the vet to have Lucy's infection lanced. Both of the bites were on Lucy's neck.
For the past few months I leave Cosmo, who is almost one year old, outside during the daytime. If the weather is permitting. If not he is in the spare bedroom. At night Lucy is in the bedroom and Cosmo roams the house. I am hoping that someone has some suggestions on how to create a relationship with these cats where they will TOLERATE one another.

To be honest, my advice would be to re introduce them all over again. Seperate them, and then gradually let them get used to each others scents again.
However, I am actually battleing the same thing right now. I am pretty good with cat behavior, but these 2 have me stumpped. They fight like you wouldnt beleive! I have tried everything under the sun. For now, seperation is my answer and then I am going to drugs!!
I know it's fairly common among multi cat households. All of them with a different personality, you are bound to have an uneven match.

I'm going through the same thing with my three cats after 3 years of getting along great. Rocky & Thumper are going after my older cat Julian. It got so bad we called the vet. He had us isolate Julian from the other two for a week. We now bring them together on supervised visits. The vet told us to use a spray bottle with water on the problem cats when they go after Julian. Also to put different sounding bells on them so we know where they are. This doesn't work real well because Thumper takes his off. We've been trying this for a week now. Julian will be in the same room but still gets upset when they move towards him although they haven't tried to attack him. We have to work on his fear now.

At another site I frequent, there was a long list of people who had this happen where it turned out the the cat being attacked was slightly ill. Apparently, when a cat becomes sick, his/her scent changes just enough to make the others believe it is an intruder. Have your vet run some blood tests or a urinalysis and see whether there might be an illness causing the problem.

Thank you for your post. I am taking Lucy to the vet on 1/8 to have her teeth cleaned. The blood work sounds good, it has been a while. I will let you know the results.

The blood work for Lucy came back negative for everything. I am going to continue trying to slowly re-introduce them to one another.