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Simple Summary: To define a healthy animal in an experimental setting or to differentiate and backup a diagnosis in cattle practice, reference intervals (RIs) in haematology diagnostics are necessary. The RIs in calves for blood cell counts, such as platelets and white blood cells, differ from RIs in adult cattle and are not widely studied. Blood results from dairy calves in the Netherlands were used to study the variation in platelet counts in young calves and to calculate an RI for platelet and white blood cell counts. In new-born calves up to six days of age, platelet counts were lower than in calves older than five days. From six days of age until 60 days of age we propose an RI platelet count of 287–1372 × 109/L and for the first 60 days of life an RI for leukocyte count of 4.0–18.9 × 109/L. Platelet and leukocyte count reference intervals (RIs) for cattle differ by age and while adult RIs are known, RIs for calves are studied less. The aims of this observational study are to evaluate variation of platelet counts of Holstein Friesian calves over the first 14 days of life and to propose RIs for platelet and leukocyte counts of Holstein Friesian calves aged 0–60 days. In a longitudinal study, 19 calves were blood sampled 17 times, in the first 14 days of their lives. Blood was collected in a citrate blood tube and platelet counts were determined. We assessed the course of platelet counts. In a field study, 457 healthy calves were blood sampled once. Blood was collected in an EDTA blood tube and platelet and leukocyte counts were determined. The RIs were calculated by the 2.5 and 97.5 percentiles. Platelet counts started to increase 24 h after birth (mean platelet count 381 × 109/L ± 138 × 109/L) and stabilized after five days (mean platelet count 642 × 109/L ± 265 × 109/L). In calves up to six days of age, platelet counts were lower than in calves older than five days. In conclusion, the RIs of platelet and leukocyte counts in calves were wider in range than the RIs for adult cattle, therefore, calf specific RIs for platelet and leukocyte counts should be used. From 6 until 60 days of age, we propose an RI for platelet counts of 287–1372 × 109/L and for the first 60 days of life an RI for leukocyte counts of 4.0–18.9 × 109/L. [ABSTRACT FROM AUTHOR]