An automated fluorescent microsphere-based flow cytometric triplex immunoassay, using the Luminex 100 flow analyzer with MultiAnalyte Profiling (xMAP) technology, was developed for the simultaneous detection of proteins from three vegetable sources as potential fraudulent adulterants in milk powder. In the final triplex inhibition immunoassay, soluble wheat proteins (SWP) and proteins from soy and pea were coupled to three different microsphere sets. A mixture of these microsphere sets was transferred to a microtiter plate well together with the sample and a mixture of three affinity-purified polyclonal antibodies raised against the proteins and labeled with a fluorophore (Alexa 532). After incubation for 1.5 h at room temperature in the dark, the fluorescence intensities on the microspheres were directly measured (no wash procedure) in the Luminex during 10 s per well (100 microspheres per set). The sensitivities of the three assays for plant protein extracts were determined as 0.5-0.6 microg/mL at 50% inhibition. For the detection of the vegetable proteins in milk powder, the samples were dissolved in buffer (0.1 g in 10 mL) and further diluted (20 times) to create a 50% inhibition at approximately 0.5% of the vegetable proteins in the total protein content of milk powder. With the help of calibration standards, prepared under conditions comparable to those for sample materials, the triplex immunoassay proved to be quantitative above 0.1%, although concentrations in high-heated milk powders were underestimated. Due to the xMAP technology, in which 100 different microsphere sets can be distinguished, this triplex immunoassay can easily be extended to detect other possible adulterants.