A rapid, simple method for the determination of serum by ICP-MS is described. Samples diluted 1:10 with deionized water are quantitated from a calibration curve determined with aqueous standards. Yttrium is used as the internal standard. Data showing detection limit, precision, and linearity are presented. Accuracy is assessed by analysis of NIST reference materials and by participation in interlaboratory performance testing (PT) programs. The performance of the method in one such PT program is discussed.