It has been proposed that milk protein gene expression in human breast carcinomas may indicate a functional oestrogen receptor mechanism, and may therefore be diagnostic of tumours responsive to endocrine therapy. Unfortunately this has not been proved, largely because of inconsistencies in the immunoassay procedures used to identify milk proteins, in particular alpha-lactalbumin, in tissue extracts or serum. Alternative procedures include the identification of milk protein mRNA by cell-free protein synthesis, and the identification of milk protein RNA transcripts by hybridization to a sequence-specific probe. Here we describe experiments using alpha-lactalbumin cDNA probes, purified using recombinant plasmids, which demonstrate that although polyadenylated and non-polyadenylated alpha-lactalbumin transcripts are present in normal human mammary tissue during pregnancy and lactation, alpha-lactalbumin transcripts are not detectable in the human tumour tissues studied. These experiments do however show that a peptide, which shares antigenic determinants with human alpha-lactalbumin is present in some breast tumour tissues.