The objective of this study was to determine whether insertion-deletion (indel) 23 bp polymorphism of the bovine prion protein (PRNP) gene differentiates the total number of leukocytes and lymphocytes and the number of virus-infected lymphocytes. The experimental materials comprised 119 Black-and-White Polish Holstein-Friesian cows. Bovine leukosis was diagnosed by an indirect immunofluorescence based on the detection of viral protein p24 in bovine lymphocytes infected with leukaemia virus (BLV). Indel23 polymorphism was determined by PCR. Blood haematological parameters (total leukocyte counts, total lymphocyte counts, and their percentages) were determined at a specialist haematological laboratory. The examined indices were analysed in three replications, at one-month intervals. It was found that indel23 polymorphism significantly differentiated blood leukocyte counts and the total number and percentage of lymphocytes. Cows with the 23 bp del/del genotype showed significantly higher leukocyte and lymphocyte counts than animals with the remaining two genotypes. Higher values of the analysed haematological parameters noted in homozygotes with 23 bp deletion are similar to the values reported in cows affected by persistent lymphocytosis, thus pointing to an adverse effect of this genotype on the haemopoiesis process. The variations between indel23 genotypes and the number and percentage of BLV-infected lymphocytes are less obvious and more difficult to interpret.