The resin-I5 column developed in our laboratories rendered aqueous suspensions containing up to 5 X 10(4) cysts of Giardia muris or Giardia lamblia per ml incapable of excystation. The inhibition of excystation was effective at both 4 and 25 degrees C. The addition of Na2S2O3 to column eluates containing cysts appeared to partially reverse the disinfectant action, and the reversal was more pronounced at 4 degrees C than at 25 degrees C. In contrast, the rapid removal of cysts from the column eluates by centrifugation and filtration or the use of other reductants, notably cysteine and glutathione, did not similarly reverse the disinfectant properties of the column. Based on these data, we suggest that the disinfecting agent is acquired by the cyst in its passage through the resin column and that either the disinfecting agent or its reaction can be partially and specifically neutralized by Na2S2O3. We hypothesize that the time between disinfectant acquisition and activity is a function of the thickness of the Giardia cyst wall and consequently takes longer at the lower temperature. Nevertheless, resin-I5 appears to inactivate a larger number of cysts in a shorter period of time with lower residual halogen levels than do agents of other published methods.