We present the results of a study in which biomolecular interaction analysis (BIA, Biacoretrade mark 2000) was combined with mass spectrometry (MS) using entire "on-a-chip" procedure. Most BIA-MS studies included an elution step of the analyte prior MS analysis. Here, we report a low-cost approach combining Biacore analysis with homemade chips and MS in situ identification onto the chips without elution step. First experiments have been made with rat serum albumin to determine the sensitivity and validation of the concept has been obtained with an antibody/antigen couple. Our "on-a-chip" procedure allowed complete analysis by MS/MS(2) of the biochip leading to protein identifications at low femtomole to sub-femtomole levels. Using this technique, identification of protein complexes were routinely obtained giving the opportunity to the "on-a-chip" processing to complete the BIA-MS approach in the discovery and analysis of protein complexes.