I have a male cat he is 1 yrs old, just recently got a female cat she is 1 and a half yrs old, both have been done, I've slowly been introducing them both but my male cat keeps hissing and growling at my new cat I'm worried that he will not accept her, have been doing this for a few days now and no change, don't know what else to do.

Not all cats are willing, or able, to be friends. Keep them separated when you're not around, and keep an eye on them when you are. They may warm up to each other.

Make sure they have their own space, especially right now. Separate food dishes and litter boxes. Separate places to lounge and sleep. It may help cut down on territorial issues.

A few days means nothing. People a lot of times think that just because some hissing and growling is going on that it means something is wrong, or that they'll never get along.

This is just a typical reaction for a cat. It could last a while. Even if it stops, it can start again. My newest cat has now been here two years and even now there's an occasional hiss (especially if food is involved).

I feel pretty strongly that they should be allowed to work things out for themselves. Keep them separated when you're in bed and when you're gone. (i.e. when you can't keep watch out of the corner of your eye.) Just be patient and let it happen naturally - it can take time.