HnRNP proteins are abundant nucleoplasmic pre-mRNA-binding proteins which have important roles in the biogenesis of mRNA. Although hnRNP proteins have been extensively characterized in cultured cell lines, little is known about their expression in animal tissues. Here, we have undertaken a systematic survey of the expression of major hnRNP proteins in mouse tissue using specific monoclonal antibodies. Immunohistochemical staining demonstrated that hnRNP proteins C, L, and U were localized to nuclei in all tissues examined. However, cytoplasmic expression of hnRNP A1, D, F/H, and K was also detected in several tissues, suggesting that these proteins have roles in the cytoplasm as well as the nucleus. Importantly, the relative amounts of different hnRNP proteins varied among cell types. This was especially striking in neuronal and reproductive cells. In the brain, certain neuronal cell types contained more hnRNP proteins than glial cells, perhaps reflecting increased levels of neuronal transcription and RNA processing. In the ovary, oocytes contained exceptionally high concentrations of hnRNP proteins as compared to follicular and stromal cells. In the testis, the expression of hnRNP proteins was generally high and was found to be tightly regulated during spermatogenesis. Specifically, hnRNP A1 was highly expressed only in early spermatogonia and absent in later stages. These findings demonstrate that hnRNP proteins do not exist in a fixed stoichiometry across different cell types. Furthermore, as the relative amounts of pre-mRNA-binding proteins (e.g., A1 and ASF/SF2) can affect alternative splicing patterns, the variations that we have observed could profoundly affect cell-specific gene expression.