It is unclear whether differences in serum IgG concentration of calves at 24 h (acquired passive immunity) influence the onset of active IgG production or the age at which serum IgG concentrations normalize. Our objective was to monitor changes in serum IgG and total protein (TP) concentrations in calves fed to achieve high (H) or low (L) passive immunity. Newborn Holstein calves (initial BW = 42.2 ± 4.7 kg) were individually fed a colostrum supplement (n = 23; 50 g of IgG/feeding) or a colostrum replacer (n = 22; 150 g of IgG/feeding) at 0.5 ± 0.22, 6.6 ± 0.46, and 12.9 ± 1.94 h of age. Blood was collected and serum analyzed for IgG using radial immunodiffusion and total protein using optical refractometer every 7 d from wk 0 (2-3 d of age) to wk 8. Calves were vaccinated with InforceTM 3 (Zoetis) on arrival, Bovi-shield Gold® 5 (Zoetis) at 2 and 6 wk and Presponse® HM (Boehringer Ingelheim Vetmedica) at 5 and 8 wk, as prescribed by a veterinarian. Data were analyzed as a completely randomized design using repeated measures ANOVA. Serum IgG (g/L) and total protein (TP; g/dl) concentrations at 24 h of age were 19.4 ± 0.63 and 4.07 ± 0.076, respectively, in calves fed H, and 8.52 ± 0.62 and 3.32 ± 0.074, respectively, in calves fed L. Serum IgG and TP were affected by a week x treatment interaction (P < 0.001). Serum IgG concentration in calves fed H declined to 13.7 g/L at wk 3, then increased to 20.7 g/L at wk 8. Serum IgG in calves fed L declined to 6.3 g/L at wk 1, then increased to 20.6 g/L at wk 8. By wk 7, serum IgG concentration were similar (P > 0.05). Serum IgG was < 10 g/L for 0.3 and 4.9 wk in calves fed H and L, respectively. Total protein concentrations were lower in calves fed L from wk 0 to 6 (P < 0.01); thereafter, differences were not significant. Temporal changes in serum TP and IgG concentrations were independent. Active IgG synthesis was dependent on age of calf and acquisition of passive immunity. Calves with lower serum IgG concentrations at wk 0 began producing IgG at an earlier age and produced more IgG, so that by wk 7, circulating IgG concentrations were similar to those in calves with successful passive immunity.