The defence against infection in high‐yielding dairy cows is correlated with the number and function of circulating neutrophils and depends on their production in bone marrow. Therefore, the DNA content of isolated bone marrow cell suspensions from 7 calves, 7 cows and 14 humans was assayed by flow cytometry. Bovine sternal bone marrow samples were collected within 30 min of death, and human marrow samples were collected by sternal puncture and aspiration. Mononucleated cells were isolated by gradient centrifugation. In the bone marrow samples from calves and cows, 35 ± 2.6% and 31.8 ± 1.5% of the isolated bone marrow cells respectively were in the S/G2/M‐phase. The difference between calves and cows was not significant. In the human samples, only 12 ± 0.8% of the cells were in the SIG2/Mphase. A significant (P < 0.001) difference was observed between the two species. These results indicated that the proliferative, in activity of haematopoietic cells is significantly higher in cattle than in humans.