Local expression of haptoglobin (Hp) mRNA in tissue homogenates from different sites of the bovine mammary gland has recently been established. Besides alveolar and stromal tissue, blood cells are one potential cellular location of Hp expression. The aim of this study was to investigate whether bovine leukocytes might contribute to the Hp mRNA transcripts found in the mammary gland after their migration into this organ. For this purpose RT-PCR was carried out with total RNA extracted from leukocytes in blood and from somatic cells in milk of three dairy cows. In comparison, human leukocytes of four donors were examined. Both bovine blood and milk somatic cells showed distinct signals for Hp mRNA in all samples in contrast to human leukocytes where Hp mRNA was detected in only two out of four donors after reamplification. In conclusion, the presence of Hp mRNA in blood derived leukocytes indicates that these cells contribute to the Hp mRNA transcripts found in the bovine mammary gland. It remains to be clarified whether other cells within the mammary gland are also capable of Hp mRNA synthesis.