In many eukaryotes it is thought that cell proliferation is regulated at a point in G1 close to the initiation of DNA synthesis. Hartwell and his colleagues have shown such a point in G1 phase in the budding yeast, Saccharomyces cerevisiae, defined by the cdc 28 mutation. He has termed this point 'start' and showed that for cells to proceed beyond start, initiate DNA synthesis and produce a bud, various conditions must be met. Two of these conditions are the presence of adequate nutrients in the medium and attainment of a critical size. We identify here some of the genes controlling start by isolating mutants which are altered with respect to the conditions in which start occurs. Two types of mutant have been isolated. One results in bud initiation when the parent cell is only half the size at which bud initiation occurs in wild-type cells. Such mutants define a single gene, whi-1, and they are apparently analogous to the size mutants isolated by Nurse and his colleagues in Schizosaccharomyces pombe. A second type of mutation affects a second gene, whi-2, which is involved in the mechanism whereby cells arrest in G1 in stationary phase. whi-2- cells growing exponentially initiate buds at the same size as wild-type cells. In stationary phase, however, whi-2- cells growing exponentially initiate buds at the same size as wild-type cells. In stationary phase, however, whi-2- cells, unlike wild-type cells, are predominantly budded and are smaller than wild-type cells.