The ability to replace genes coding for cellular proteins with DNA that codes for fluorescent protein- tagged versions opens the way to counting the number of molecules of each protein component of macromolecular assemblies in vivo by measuring fluorescence microscopically. Converting fluorescence to absolute numbers of molecules requires a fluorescent standard whose molecular composition is known precisely. In this report the construction, properties, and mode of using a set of fluorescence calibration standards are described. The standards are based on an icosahedral virus particle containing exactly 240 copies of one of seven different fluorescent proteins. Two applications of the fluorescent standards to counting molecules in the human parasite Toxoplasma gondii are described. Methods for improving the precision of the measurements and avoiding potential inaccuracies are emphasized.