Ability to secrete the K1 (or K2) toxin protein and immunity to that toxin [the K1 (or K2) killer trait] are determined by a double-stranded (ds) RNA, called M1 (or M2), whose replication and maintenance depend on at least one of the larger (L) ds RNAs and 29 chromosomal genes, called MAK genes (maintenance of killer). The location of the MAK8 gene near TCM1 (trichodermin resistance) on the yeast map suggested the possible identity of these two genes. Of six independently isolated tcm1 mutants, five were clearly mak-, and the sixth was weakly mak-. In each case, the mak- phenotype and the trichodermin-resistant phenotypes cosegregated in meiosis and showed the expected tight linkage to pet17. The mak- mutations in the trichodermin-resistant strains did not complement mak8-1, indicating that MAK8 and TCM1 are the same gene. The mak8-1 mutation does not make strains resistant to trichodermin, and one tcm1 mutation is only slightly mak-. Whereas tcm1 mutants lose M1 or M2 ds RNA, they do not lose L ds RNA. Because TCM1 codes for ribosomal protein L3 [Fried, H. M. & Warner, J. R. (1981) Proc. Natl. Acad. Sci, USA 78, 238--242], we conclude that ribosomal protein L3 is involved in the replication and maintenance of M ds RNA. Mutations in cyh2 or cry1, producing resistance to cycloheximide and crytopleurine due to mutant ribosomal proteins, do not produce a mak- phenotype. In analogy with bacterial ribosome assembly mutants, yeast low-temperature-sensitive (lts) mutants may have defective ribosomes. We thus examined mutants for an effect on the killer system. An lts5 mutant, unable to grow at 5 degrees C, also has a mak- phenotype (at 30 degrees C) that cosegregates in meiosis with the lts- phenotype. Mutations in seven other lts genes do not result in the mak- phenotype.