Stability of NHS esters of mPEG5k under accelerated conditions. Effects of two conditions were examined consecutively. In the first stage, samples in sealed bottles were kept at 37°C for one week. In the second stage, bottles were opened and samples were exposed to saturated humidity at 37°C in a closed cell culture incubator. At the same time in every indicated day for both stages, samples in bottles were dissolved with equal volumes of DMF to estimate molar equivalent of NHS esters under stated conditions.