Sixty-eight new conditional cell cycle mutants have been isolated on the basis of their terminal cellular morphology ('dumbbells'). Fifteen mutants falling into nine complementation groups, were grossly defective in DNA replication and have been assigned the provisional gene symbol dbf (for dumbbell former). Dbf1 and 2 stop DNA synthesis immediately on transfer to 37 degrees C and are presumably defective in enzymes required for polymerization. Neither, however, possess a thermolabile DNA polymerase A or B. Dbf3 and 4 show a pattern of synthesis consistent with their being deficient in initiation of DNA synthesis. This is confirmed in the accompanying paper. The remaining mutants are deficient in the synthesis of RNA as well as DNA. Indeed the four members of one complementation group are allelic with rna3, one of the group of mutants originally isolated as defective in RNA synthesis, and which do not exhibit a cell cycle phenotype. A re-examination of this group of mutants, however, showed the bulk of them also to be defective in DNA synthesis. Furthermore, in preliminary experiments rna3 and our four new alleles of it, together with rna6 and dbf5 and 6, showed enhanced spontaneous mutation frequency.