AN EMB145 ON TKOF CLB AT 2500 FT DECLARED AN EMER AND RETURNED TO THE DEP ARPT DUE TO CABIN ATTENDANT RPT OF SMOKE IN THE CABIN.

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AFTER TKOF; WE WERE CLBING THROUGH 2500 FT; 'LAVATORY SMOKE' CAME ON EICAS; AND THE FLT ATTENDANT CALLED UP TO SAY THAT THERE WAS SMOKE IN THE CABIN AND THAT SHE COULD NEITHER SEE ANYTHING OR BREATHE. I ADVISED HER THAT WE WOULD BE LNDG IMMEDIATELY. WE DECLARED AN EMER WITH APCH; THE FO DONNED HER GOGGLES AND OXYGEN MASK AND RAN THE EMER CHKLIST FOR SMOKE/FUMES. I CONTINUED TO FLY THE ACFT AND TALK ON THE RADIO; THE FO MADE A PA TO THE PAX ADVISING THEM THAT WE WOULD BE RETURNING TO LAND. WE WERE VECTORED TO THE ILS RWY 27L AND LANDED. ONCE ON THE GND AND CLR OF THE RWY; WE CHKED WITH THE FLT ATTENDANT AND SHE SAID THE SMOKE SEEMED TO CLR AND SHE WAS ABLE TO BREATHE; SO WE DECIDED TO CONTINUE TO A GATE TO DEPLANE THE PAX. THE FO AND FLT ATTENDANT BOTH DID AN EXCELLENT JOB; WE WERE ONLY IN THE AIR FOR ABOUT 8 OR 9 MINS TOTAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED IT WAS INITIALLY BELIEVED DEICING FLUID HAD LEAKED INTO THE AIR CONDITIONING SYS AND CAUSED THE SMOKE IN THE CABIN. LATER ADVISED BY A LEAD TECHNICIAN THE SMOKE WAS CAUSED BY A FAILED BLEED VALVE; BUT NO CLARIFICATION ON WHAT STAGE VALVE AND WHAT ENG POS.