RETURN AND LAND BY A B737-300 FLT CREW WHEN THE MASTER CAUTION AND AFT CARGO DOOR WARNING LIGHT IS NOTED AFTER DEP TUL; OK.

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WE ARRIVED AT THE TUL ARPT FROM HOTEL FOR DAY 3 OF A 3-DAY PAIRING. AFTER PREFLTING THE ACFT AND LOOKING FOR STROLLER FOR PAX THE GND GUYS INFORMED ME THAT THEY COULD NOT OPEN THE FORWARD CARGO BIN DOOR. I INFORMED THE CAPT AND THE DECISION WAS MADE FOR CONTRACT MAINT. AFTER A SHORT DELAY; THE DOOR WAS OPENED AND THE BAGS WERE DOWNLOADED AND UPLOADED FOR OUR FLT. OTHER THAN THE FORWARD CARGO DOOR ALL OUR OTHER PREPARATIONS FOR FLT INCLUDING PUSHBACK AND TAXI OUT WERE NORMAL. RIGHT AFTER TKOF AT APPROX 500 FT AGL WE GOT A MASTER CAUTION AND AN AFT CARGO DOOR LIGHT. SINCE WE WERE STILL IN THE TFC PATTERN AND KNEW WE HAD A LINE OF WX TO NAV INTO DEST AND THE ASSOCIATED TURB WE ELECTED TO RETURN TO TUL AND HAVE THE DOOR CHKED OUT. EVEN THOUGH WE OBVIOUSLY HAVE A PROC FOR THIS; GIVEN ALL THE ABOVE CIRCUMSTANCES I FEEL WE EXERCISED OUR JUDGEMENT CORRECTLY. THE IMPACT ON CONNECTING FLTS IN DEST WAS MINIMAL DUE TO THE WX. SUPPLEMENTAL INFO FROM ACN 612995: THE CREW WHO BROUGHT THE ACFT IN RPTED THAT ALL WAS FINE. SHORTLY AFTER RAMP PERSONNEL RPTED DIFFICULTY WITH THE FORWARD CARGO DOOR. AFTER SEVERAL MINS ATTEMPTING TO OPEN THE DOOR WITH NO SUCCESS; CONTRACT MAINT WAS CALLED. THE MECH SPOKE WITH MAINT CTL AND WAS ABLE TO OPEN THE DOOR. WE COULD HAVE CONTINUED WITH THIS CAUTION LIGHT; BUT WITH THE PRIOR EVENT WE FELT THAT A RETURN TO THE ARPT WAS THE BEST DECISION. UPON RETURNING TO THE GATE THE CARGO DOOR LIGHT EXTINGUISHED. AFTER PARKING THE ACFT A JUMPSEAT RIDER AND COMPANY EMPLOYEE VISUALLY CHKED THE CONDITION OF THE DOOR AND DETERMINED THAT IT WAS FINE. AFTER CALLING DISPATCH AND MAINT THE CARGO DOOR WARNING SYS WAS MEL'ED.