Different organ perturbation and multiple complications might occur after cardiopulmonary bypass (CPB). A variety of solutions might be used for pump priming with different advantages and disadvantages. The advantage of fresh frozen plasma (FFP) inclusion in pump prime has been shown in post-CPB coagulation management. Acquired hypogammaglobulinemia is the disadvantage of albumin (ALB) pump prime. Our aim was to assess the impact of FFP prime on the post-pump serum level of immunoglobulin G (IgG) and its subclasses. Fifty-six patients under the age of 5 years old who were scheduled for cardiac surgery on CPB were randomly primed with FFP or ALB. Any innate or acquired immune deficiency was considered as exclusion criteria. The pre-CPB and 24-hour post-CPB collected blood samples were analyzed by the nephelometric method for the plasma level of IgG and its four subclasses. Twenty-two patients (mean age and weight of 13 months and 6.8 kilograms) in the ALB prime group and 26 patients (mean age and weight of 15 months and 8.1 kilograms) in the FFP prime group completed the study. Using paired t-test and repeated measures ANOVA test, patients in the ALB prime group had a significant drop in the post-CPB serum level of total IgG (597±138 mg/dL to 379±179 mg/dL, P value <0.001) and its two subclasses of IgG1 and IgG3. In contrast, there was a slight elevation in the serum level of total IgG (549±207 mg/dL to 630±180 mg/dL, P value =0.008) and its two subclasses of IgG2 and IgG4 in patients who had FFP prime solution. In conclusion, compared to the ALB prime solution, FFP inclusion in prime could hamper the pediatric post-CPB induced hypogammaglobulinemia.