This chapter presents a general procedure for the on-chip detection and quantitation of low-molecular-weight recombinant proteins in transgenic plant crude extracts by surface-enhanced laser desorption ionization time-of-flight mass spectrometry (SELDI-TOF MS). A protocol is first described to detect the protein of interest in crude protein extracts of transgenic plant lines, by differential protein mapping against similar extracts from a control, nontransgenic line. A complementary protocol is then presented to generate a standard curve with the SELDI system, allowing the protein to be quantified in different transgenic lines. Overall, this procedure may be carried out within a few hours, without the need for prior purification or enrichment of the recombinant protein.