In order to study the development of blood serum Ig levels in beefcalves from birth to 3 months of age and factors affecting these development, an experiment was conducted with 84 cow-calf pairs for a period of 6 months. The results obtained from the experiment are summarized as follows. 1. Calves show the highest serum IgG and IgM levels 36 hours post partum and the highest IgA level 24 hours post partum. These levels are, on average, 34.6%, 57.2% and 43.8% respectively of the respective average Ig levels of colostrum. 2. IgA and IgM levels of serum in calves are lowest at 4 weeks of age and rise again thereafter, while the IgG level decreases until 3 months of age. 3. At 3 months of age, IgA and IgG serum levels in calves are nearly equal to the respective levels in their dams, while the serum IgM level in the calf at this age reaches only a third of that in the dam. 4. Factors affecting the calf serum Ig levels: 1) Time of first colostrum intake: The earlier calves the first colostrum suckle, the higher the three Ig levels in their blood serum are. 2) Quantity of colostrum suckled: It appears that calves which suckled more than 1.2 Kg of colostrum at first suckling show higher serum Ig levels in all classes than those which suckled less than this amount. 3) Quality of colostrum: The higher colostral Ig levels are, the higher Ig levels in blood serum of calves during the first days post partum are. 4) Sex: There are no differences in serum Ig levels between male- and female calves at 24 hours of age and thereafter.