A male baby was born in a hospital. After 12 hr of bih, baby was found to be pale. His serum total billirubin level was 20 mg/dl and unconjugated bilirubin was 15 mg/dl, after 36 hr of bih, Hemoglobin was 14 g/dl, and reticulocyte count was high. On peripheral smear, nucleated RBC's and spherocytes are seen. Best possible diagnosis is?