The electrospray mass spectra of several standard proteins were recorded and their molecular weights determined. These were compared to Literature values obtained by other laboratories. The agreement was quite good. Seven recombinant growth hormones were then investigated by this technique. The determined molecular weights were in agreement with the theoretical values. Differentiation between six of the seven analogs could be made on the basis of the molecular weight determinations. Two of these analogs differed in molecular weight by only 1 Da and the accuracy of the mass measurements was not sufficient for distinguishing these two homologous proteins. The influence of solvent on the mass accuracy determinations was studied. It appears that solvents form clusters with the proteins in the electrospray ionization method. These clusters broaden the peaks corresponding to the multiply charged ions of the proteins and make the detemination of centroids more difficult.