Can I replace BSA with fetal calf serum or newborn bovine serum to supplement PBS buffer? How much of each substitute would be equivalent?

In our experience, BSA should be your first choice when supplementing the PBS buffer. First, BSA is a well-defined single compound, as opposed to fetal calf serum (FCS), which is composed of many undefined proteins and molecules. In addition, using FCS may increase the concentration of divalent cations, such as Ca2+ and Mg2+, in the buffer, and in some cases, this could increase clumping of your sample. However, if you cannot use BSA, we recommend using FCS at a final concentration of 5%, which is typical for flow cytometric staining procedures.