Kluyveromyces lactis killer toxin causes sensitive strains of a variety of yeasts to arrest at the G1 stage of the cell cycle, and to lose viability. We describe here the isolation and characterization of a class of recessive mutations in Saccharomyces cerevisiae that leads to toxin resistance and a temperature-sensitive phenotype. These mutant cells arrest growth at 37 degrees C with a characteristic phenotype of elongated buds. Cloning of the gene complementing these defects revealed it to be CAL1, coding for chitin synthase 3 activity. Calcofluor staining of the mutant cells indicated that chitin is absent both at 23 degrees C and 37 degrees C. Given that the CAL1 activity is responsible for the synthesis of most of chitin in yeast cells, and that in its absence the cells are viable but resistant to the killer toxin, our results strongly suggest that chitin might represent the receptor for this killer toxin.