Partial chemical analyses of large (d > 100 mu m), single fluid inclusions have been obtained with a laser microprobe. Weight concentration ratios of Na, Mg, Mn, and Cu relative to Ca in inclusion fluids within quartz from the Finlandia vein, Colqui, Peru, from Casapalca, Peru, and from the OH vein, Creede, Colorado, were determined within a factor of 2 to 5. Individual cation concentrations were determined with an uncertainty of a factor of 3 to 7 by combining the calculated weight ratios with the measured fluid inclusion salinities. The concentration of Ca ranged from a few hundred ppm to a few thousand ppm, the concentration of Mg from a few tens to a few hundred ppm, and the concentration of Mn from a few ppm to about 100 ppm.Cu was detected in very few of the spectra of natural fluid inclusions. In spectra in which Cu was observed, its presence was probably due to contamination. The upper limit of the concentration of Cu in the analyzed inclusion fluids is estimated to have been in the range of 1 to 40 ppm at Colqui, 1 to 50 ppm at Casapalca, and 1 to 25 ppm at Creede. These results are consistent with Crerar and Barnes' (1976) solubility measurements of chalcopyrite in the presence of pyrite, which indicate that in the hydrothermal solutions of stage II Au-Ag mineralization in the Finlandia vein at Colqui and in the OH vein at Creede the concentration of Cu was probably between 1 and 10 ppm.