RNA complexity analyses of total cellular polyadenylate-containing RNA isolated from lactating human breast tissue, human breast tumor tissue, and a mixture of established cell lines of mammary origin demonstrate extensive homology between the tissue RNA populations but suggest a decrease in the complexity of cell line nuclear RNA populations, with the exception of an early-passage MCF-7 cell line. Cell-free protein synthesis and two-dimensional gel electrophoresis also show quantitative and qualitative differences in gene expression between human mammary tumor tissue and reduction mammoplasty or established mammary cell lines of early and late passage number. The results demonstrate a major role for transcriptional and posttranscriptional mechanisms in the regulation of gene expression in the human mammary gland and show that studies on mammary gland gene expression using established cell lines of mammary origin reflect only in part gene expression in normal lactating human breast or breast tumor tissues.