In replacing a broken drain pipe in bathroom sink, I discovered that the original plumber used plumber's putty to seal the join between the 1-1/4" metal drain pipe and the 1-1/2" PVC pipe leading to the PVC P trap. I know not to use plumber's putty on PVC, but since two different sized pipes were used, what are my options? The metal drain pipe is about 3" down into the PVC so it drains fine but is not sealed. It's noisy and I'm concerned about the gas buildup from the trap smelling in the future.

Nice catch.

Use one of these: Fernco Inc P1056-150-125 1-1/2" x 1-1/4" Coupling

There is several transition / reducing adaptors on market that allow you to connect 1 1/4" with 1 1/2" pipes. They can be ABS, PVC, or u can use Fernco fittings as suggested by KISS above... Do not use putty... Do it the right way...

Thank you! I'm off to the hardware store.
In checking the other bathrooms in the house, I see that they were all installed this way... using plumbers putty to seal the joints between the smaller metal pipe into the larger PVC pipe. The house is only 2 years old. Would you recommend I change all the joints? What exactly can go wrong by using the putty? We don't live in the house full-time so I don't want a problem to occur while we are gone, although we do turn off the water and drain the pipes before leaving.

Sewer gas leaks into the home. The primary component is Hydrogen sulfide. All sorts of health effects. See: Safety (MSDS) data for hydrogen sulfide

Sometimes it's worth while filling drains and traps with vegetable oil which won't evaporate if a house is not going to be used for a long time. This includes the cellar drain.

Your probably connecting to galvanized pipe and without threading the rubber couplings work.

This is a suggestion that I missed: 1-1/2" Proflex® Reducing Coupling For Cast Iron, Plastic Or from Fernco

The stainless band gives the coupling some rigidity.

Grab some plumber's grease while your at it.

As Milo said, reducers on the PVC side are also possible.

Threading galvanized will just annoy you.

BridgedG, using putty to seal two pipes together is like using chewing gum to fix broken door handle.

Besides health hazards and possible leaks, it is simply wrong. Also, it is against the Code no matter where you live. .