We have characterised a Saccharomyces cerevisiae cDNA (cDNA13), originally isolated on the basis of the short half-life of the corresponding mRNA. We show here that its sequence is closely related to that of the genes encoding ribosomal proteins K37, KD4 and K5 of Schizosaccharomyces pombe. 'mRNA13' also behaves like other mRNAs encoding ribosomal proteins, in that its abundance increases sharply when glucose is added to cells grown on ethanol (nutrient-upshift), and declines when cells are subjected to a mild heat-shock. Unspliced mRNA13 accumulates when cells bearing a temperature-sensitive splicing mutation are grown at the restrictive temperature. The gene(s) corresponding to cDNA13, like other ribosomal protein genes of S. cerevisiae, thus contain an intron. Southern blot analysis indicates the presence of two separate loci related to cDNA13 in the S. cerevisiae genome. From the sequence of one of these, a complete polypeptide sequence was deduced. The first 40 amino acids are identical to those of YL6, a S. cerevisiae ribosomal protein characterised only by N-terminal protein sequence analysis. There is clear evidence within the genomic sequence for the predicted intron, and for elements similar to those that regulate expression of other S. cerevisiae ribosomal protein genes.