Bovine macrophages underwent apoptosis as a result of infection with a Mycobacterium bovis field strain. Macrophages infected with a multiplicity of infection (MOI) of 25:1 developed chromatin condensation and DNA fragmentation at 4 h and 8 h, respectively, whereas changes in chromatin condensation induced by MOIs of 10:1 and 1:1 required more time and had a reduced number of apoptotic cells. Not only infected macrophages underwent apoptosis, but also uninfected bystander macrophages became apoptotic. Increased differential expression of thymosin beta-10 was identified in M. bovis-infected bovine macrophages by differential display reverse transcriptase PCR. Phagocytosis of latex beads had no effect on the expression of thymosin beta-10, whereas bacterial suspensions upregulated thymosin beta-10 expression, suggesting that M. bovis or mycobacterial products are essential in the process. Heat-inactivated M. bovis induced a slight increase in thymosin beta-10 mRNA, whereas live virulent and attenuated M. bovis organisms increased the gene expression almost twofold. A mouse macrophage cell line (RAW 264.7) overexpressing the bovine thymosin beta-10 transgene had spontaneous apoptosis at a higher rate (66.5%) than parental cells (4.7%) or RAW cells harboring the empty vector (22.8%). The apoptotic rates of the overexpressing cells were significantly higher when compared with both the empty vector transfected (P < 0.01) and parental cells (P < 0.001). Our evidence suggests that upregulation of thymosin beta-10 in M. bovis-infected macrophages is linked with increased cell death due to apoptosis.