To study cell wall assembly, a simple screening method was devised for isolating cell wall mutants. Mutagenized cells were screened for hypersensitivity to Calcofluor White, which interferes with cell wall assembly. The rationale is that Calcofluor White amplifies the effect of cell wall mutations. As a result, the cells stop growing at lower concentrations of Calcofluor White than cells with normal cell wall. In this way, 63 Calcofluor White-hypersensitive (cwh), monogenic mutants were obtained, ordered into 53 complementation groups. The mannose/glucose ratios of the mutant cell walls varied from 0.15 to 3.95, while wild-type cell walls contained about equal amounts of mannose and glucose. This indicates that both low-mannose and low-glucose cell wall mutants had been obtained. Further characterization showed the presence of three low-mannose cell wall mutants with a mnn9-like phenotype, affected, however, in different genes. In addition, four new killer-resistant (kre) mutants were found, which are presumably affected in the synthesis of beta 1,6-glucan. Most low-glucose cell wall mutants were not killer resistant, indicating that they might be defective in the synthesis of beta 1,3-glucan. Eleven cwh mutants were found to be hypersensitive to papulacandin B, which is known to interfere with beta 1,3-glucan synthesis, and four cwh mutants were temperature-sensitive and lysed at the restrictive temperature. Finally, nine cwh mutants were hypersensitive to caffeine, suggesting that these were affected in signal transduction related to cell wall assembly.