The advent of high-capacity multi-channel analyzers allows estimation of long-term variability in serum constituents of large numbers of subjects. By frozen storage of specimens with subsequent analysis in a single machine run, long-term analytical variation may be eliminated, thus sharpening the estimates of intra-individual variation. In the present study we used the Vickers M-300 analyzer to obtain the data for such estimates from 37 male volunteers, each bled once a week for 22 weeks. Secimens were analyzed in random order to eliminate any biasing effect of analytical drift during the 4-h machine run. Ten serum constituents were measured. Storage-induced linear trends were small or negligible during the period of specimen collection. Using the ratio of average within-subject variance to the variance among subjects as a guide, serum alkaline phosphatase was found to show the greatest individuality, sodium and potassium the least. Other constitutents showed varying degrees of individuality, but for all these analytes, the usual population-based reference ranges were found to be either insensitive or irrelevant to the study of concentration changes over time within most healthy subjects. Our results generally confirmed those of smaller but comparable earlier studies.