We report on a series of experiments in Schizosaccharomyces pombe to detect the blue-colour colony phenotype associated with expression of the Escherichia coli lacZ gene. Increasing the pH in solid minimal medium to optimize blue colony colour revealed a pH-sensitive phenotype in auxotrophic strains requiring uracil and leucine as external supplements. This phenotype was observed among common S. pombe stock strains, 5-fluoroorotic acid (5-FOA)selected strains, and random genetic segregants. Growth of prototrophic S. pombe strains 972 and 975 or the adenine auxotrophic strain NCYC 1860 were unaffected by an increase in external pH. Analysis of genetic segregants from three independent crosses indicated that a single auxotrophic marker (ura4- or leu1-32) was sufficient for yeast cell-growth inhibition when the medium pH was increased above 6.6. In contrast, growth of a Saccharomyces cerevisiae strain isogenic to AH22, requiring uracil, leucine and histidine, was unaffected by changes in the pH of the medium. These observations suggest that uptake of uracil and leucine into S. pombe cells is compromised by alterations in external pH. Our results have implications for detection of the lacZ gene-encoded bluecolour colony phenotype in S. pombe, which is optimized by growth in the presence of 5-bromo-4-chloro-3-indolyl- "beta"-D-galactoside (Xgal) at pH 7.0. We discuss the conditions under which this blue-colour phenotype can be routinely observed in S. pombe.