In-depth LC-MS-based proteomic profiling of limited samples has been problematic due, in large part, to the inefficiency of sample preparation and attendant sample losses. To address this issue, we developed On-Micro Solid-phase Extraction Tip-based (OmSET) sample preparation for limited biological samples. OmSET is simple, efficient, reproducible, and scalable, and is a widely accessible method for processing ~200 to 10,000 cells. The developed method benefits from minimal sample processing volumes (1-3 μL) and conducting all sample processing steps on-membrane within a single microreactor. Here, we assessed the feasibility of using micro-SPE tips for nanogram-level amounts of tryptic peptides, minimized the number of required sample handling steps and reduced the hands-on time, and evaluated the capability of OmSET for quantitative analysis of low numbers of human monocytes.