Samples coming from biologic sources usually contain several contaminants that interfere seriously with Mass Spectrometry (MS) measurements. In this paper we report the application of MALDI-TOF MS to monitor recombinant protein expression and purification. The technique is based on the use of a C18 resin to clean and concentrate proteins in batch. The utility of this method is demonstrated for samples coming from different bacterial cultures expressing secreted and intracellular proteins ranging from 4 to 53 kDa. MALDI-TOF MS of peptide and proteins can be accomplished directly from complex bacterial cultures or from any purification step in a few minutes using the conventional stainless steel sample targets, allowing for a nearly instantaneous monitoring of the nature and integrity of recombinant expression products.