The recent discovery of human signal peptide peptidase-like 2a (SPPL2a) deficiency in humans revealed the toxicity associated with the accumulation of one of its substrates, CD74 N-terminal fragment (CD74-NTF), for certain type of dendritic cells (cDC2). We developed a two-step protocol for monitoring the accumulation of this molecule in different subsets of PBMCs and immortalized B cells, in which SPPL2a is chemically inhibited and CD74-NTF levels are then assessed by flow cytometry or western blotting. The chemical inhibition of SPPL2a has been described elsewhere, but this is the first time that this inhibition has been reported as a protocol.