Extracellular stimuli activate, on target cells, a number of signal transduction processes regulating gene expression and the function and/or synthesis of the proteins. In order to highlight the slight changes of the quantity and quality of the proteome it is essential to optimize preparative strategies able to improve the signals of the less expressed proteins and to standardize the use of high-throughput techniques useful to detect them. We describe a complete workflow useful to enrich, from PBMC protein extracts and extrapolated to their subpopulations, the low-molecular-weight proteins and peptides and to detect them by SELDI-TOF protein profiling. The described protocol can also be applied to MALDI-TOF/MS instruments in order to obtain fast, reproducible, and high-quality protein profiles.