We describe the isolation and characterisation of a full-length cDNA sequence (pZH-21) of a human ribosomal protein (rp) mRNA isolated from a cDNA library constructed from the human ZR-75-1 mammary tumour cell-line. The predicted protein is highly basic and shows 72% homology at the amino acid (aa) level with yeast rp L44. Comparative RNA blotting of ZR-75-1 poly(A)+ RNA isolated from cells cultured in the presence of the anti-oestrogen tamoxifen demonstrates the presence of a number of mRNA species whose concentration is elevated co-ordinately 5-6-fold in the presence of 17beta-oestradiol. Insulin in the presence of tamoxifen, also enhanced rp mRNA levels suggesting increased levels are a reflection of cell proliferation as opposed to specific hormonal regulation. Genomic analysis demonstrates the presence of a family of related human sequences, and homology with rat and guinea pig rp genes, but not yeast DNA. The conservation of rp aa sequence, in the absence of detectable homology at the nucleotide (nt) level, points to an important common functional role of the L44 protein in ribosome structure and function in man and yeast.