Painting - cleaning paintbrushes

I need help cleaning some paintbrushes, in the hopes that they don't need to be thrown away. My husband did some painting (latex flat and semi-gloss, with natural bristle brushes) and left the brushes sitting without cleaning them for about 5 days. I've tried hot water (as hot as my hands can stand) and dish soap; the brushes now flex, but there is still a lot of paint dried on the bristles. Any suggestions how I might revive them, or if I should simply give up on this one?

I found a product called Save-A-Brush, it is a powder and mixed with water, I tried it (skeptically) on a brush that had been left out with oil based paint dried to it for months, the next morning it was clean and like brand new, of course other than the dried paint the brush was a good one, I was amazed. I haven't seen it in a while, your local paint stores should have something simuliar if not the same product.

The product is still available. Mfg by Sav-o-Gran company I think. Follow directions!! Buy a brush comb while you're at it, if you're gonna be doing much painting. By the way, natural bristle brushes aren't intended for use with water-based paints. Get a good quality polyester nylon or nylex bristle brush instead.

my best
Sonnie

I appreciate the suggestions; I will see if I can find some of the stuff and give it a try. As for the type of bristle, he learned that after painting; thanks for confirming the information, for the benefit of next time!

For cleaning paint brushes I always use lestoil full strenth, it really does the job. Lestoil can be purchased at most supermarkets. You'll find it in the housecleaning aisle. For very stiff brushes you'll need to soak and work the bristles for a couple of days. wish you success.