14Efficient dairy-beef production relies on good quality of purchased calves, defined by breed, health, and growth 15 characteristics. Several management factors, such as commingling of calves and large group size, predispose 16 calves to diseases. Acute phase proteins are sensitive detectors of calf diseases. We studied the associations 17 between group size, serum acute phase proteins, immunoglobulin G (IgG), calf morbidity and growth of dairy-18 beef calves in a random field trial in a calf-rearing unit in Finland. 19The randomized trial was carried out at a calf rearing unit, where approximately 80 dairy or crossbred calves 20 were allocated either into a single group of 40 calves or into four groups of 10 on arrival at the calf-rearing 21 unit (at age 24.1 SD ± 9.2 days). The study was carried out on 6 arrival batches: 476 calves. Calves were 22clinically examined and blood sampled on arrival (day 0), and haptoglobin (Hp), serum amyloid A (SAA), 23 albumin and IgG were determined. Calves were weighed on arrival (day 0, average age 24.1 days), at the end 24 of the milk feed period (day 49), at approximately 200 days of age and at slaughter (carcass weight) at 15-18 25 months of age. During the rearing calves were observed by the farm workers and treated, if necessary, 26 according to predetermined instructions of the veterinary surgeon. All NSAID and antimicrobial treatments 27 were recorded and used as morbidity indicators in statistical analysis. 28There were no differences in the numbers of antimicrobial treatments or growth among the groups. The 29 majority (84.1%) of antimicrobial treatments were used against respiratory tract infections. Higher 30 concentrations of albumin and IgG on arrival extended the time before the first and the second antimicrobial 31 treatments. Complex relationships between group size, morbidity, concentrations of serum acute phase 32 proteins and IgG at arrival, and growth of calves were explored. Group size of 10 calves did not protect calves 33 from respiratory tract infections, when the small groups were sharing the air space with a large group. An 34 increased SAA concentration on arrival was associated with poorer average daily gain at two rearing periods 35 and with lower carcass weight at slaughter. 36Serum proteins could be valuable health indicators for purchased calves because they have numerous and 37 variable associations with health and growth. The mechanisms that connect increased SAA concentration and 38 poorer average daily gain over the long term remain unclear. 39