I've been here before about this. Was told to put baking soda, vinager, boil out drain and it should be fine... no such luck! It never did help and I'm at my wits end! This seemed to start about the same time as we had a new electric heat pump put in. The a/c is in a closet right next to the bathroom. I believe the tub and the a/c drain into the same pipe. Anyway... the most horrific smell of mold comes out of the drain. We have to keep the drain closed all the time. The only time we smell it is when the drain is open. Does anybody have any ideas at all as to what is going on here? We are on a concrete slab and scared to call a plumber for fear he will tell us we have to take the bathtub out and break up the foundation. Don't you think a leak would smell even with the drain closed? The smell just about takes your breath away, I know it can't be healthy. Anybody?? Please??