From: Rabbi Mordechai Leib Winkler
To: Rabbi Moshe Kahana
The responsum discusses a Braitah in Niddah 47a concerning dough mixed with terumah leaven, where Rabbi Meir and Rabbi Yehuda obligate challah, while Rabbi Yehuda, Rabbi Yose and Rabbi Shimon exempt, and questions Rabbi Yehuda's opinion given his stance that a mixture of the same kind is not nullified.
Source: Yoreh Deah I §214