In some clones of Paramecium caudatum (to be referred to as 'resistant') infectious forms of Holospora undulata bacteria which are capable of infecting the micronucleus were lysed within 3-4 h after invasion of the micronucleus. After microsurgical transplantation of the micronucleus of non-resistant into resistant paramecia (without removal of the recipient cell's micronucleus), bacteria were lysed in both micronuclei of the heterokaryon cell. They were not lysed in either micronucleus when the micronucleus of a resistant cell was transplanted into a non-resistant one. After UV-microirradiation or microsurgical removal of the macronucleus from a resistant cell, lysis did not occur. The data suggest that the micronucleus, as an independent morphological structure, cannot by itself initiate bacterial lysis. On the basis of these results it is clear that the process of H. undulata lysis in the micronucleus of P. caudatum is mediated by the activity of the macronucleus.