Help Please? My old cat hates my new cat!

I've had a kitten named Ambellina since May of 2005. I got her when she was about 1 month old, and ever since we got her shes been our little princess. We also had a dog, Skippy, but they got along just fine ever since they first met. As Ambellina has grown, shes become sort of agressive, depending on who shes playing with. She comes to me when she needs someone to pet her, but she goes to my mom to play, and usually she gets very agressive when they are playing.
Anyhow, about a week ago my mother rescued a stray who was freezing outside. We named her Mrs. Teacup. Shes very gentle and friendly. She isn't a burden at all. When she first got here, she'd hide under my bed and only come out to use the litterbox or to eat. We've kept Ambellina and Mrs. Teacup apart since mrs. teacup arrived.
Recently, Mrs. Teacup got brave and decided to leave my room. When she got to the kitchen, at first Ambellina would stare at her very carefully. Just tonight, I caught ambellina growling and hissing at Mrs. Teacup. mrs. Teacup doesn't do much. She just walks away.
I feel as though Ambellina is making mrs. teacup feel very uneasy and I don't like that. I was wondering if anyone knows of any methods I can use or steps I can take to make them get along?

Thanks so much!

They sound fine to me. Hissing, growling and posturing is normal for newcomers. Unless they are midair leaping and locked in battle, you truly do not have much to worry about.

I agree - it sounds very normal to me. I went through it when I introduced my Cedar to Tipsy and now they are the best of friends. Give it some more time before you start to get really worried about it. There is alot of great introduction advise on here about introducing a new cat to an old cat... check this link out
http://www.thecatsite.com/forums/showthread.php?t=67321

GOOD LUCK!! keep us updated, but I have no doubt in the coming weeks you will see a great change in them

I think your cats are acting perfectly normal. I think if you give it time they will become friendlier. It took my cats a long time to become friends but eventually they did.

I also think everything is going ok. It took my cats 3 weeks to even like each other a little bit. They were separated for 3 weeks, and after that we had our resident cat fitted with soft-paws, and I invested in "comfort zone" which I swear by.

I LOVE "comfort zone" and "soft paws" they both work great...and I have not any problems scince they finally met. There was some initial hissing and growling. But scince they only seen each others noses through the crack of the door for 3 weeks, they were very interested in each other.

Now the only problem I have is our resident cat does not know how hard he can play with the kitten. He bites a little too hard. But when the kitten cries, he usually backs off.

Ambellina is young enough, she should adjust just fine with time. By the way, I think Mrs. Teacup is such a cute name!!! Please keep us informed and post pics!

Its only been a week just give them time. It took 2 months before we were able to leave Huggy unattended with our other girls. Whereas taralen it took two weeks. So just give them time and keep an eye. There will be hissing what you described sounds perfectly normal. Either your cats will get on or they will put up with each other in the end.

Dx