We set a new upper limit of 1.7 x 10(-11) to the probability that two spin-
0 O-16 nuclei are in forbidden exchange-antisymmetric states. testing the v
alidity of fundamental principles of quantum mechanics. A newly designed di
fference-frequency spectrometer is used to look for the existence of these
states. The experimental test was performed by tuning the spectrometer acro
ss a molecular transition of (CO2)-C-12-O-16 belonging to the rovibrational
band at 4.25 mum, connecting the forbidden states (00(0)0, J = 25) and (00
(0)1, J = 26).