1. Plasma aspartate transaminase and alkaline phosphatase activities were measured weekly for approximately 30 weeks in a group of healthy males and females. 2. The magnitude of variation of the enzyme activities within any individual was studied with respect to four criteria: (a) the individual's normal range, (b) the normal range calculated for each sex separately, (c) the clinically accepted normal ranges, (d) the distribution of a derived term which excluded the variation of the predominant enzyme activities between individuals, and the between-batch variation. 3. The main conclusions were: (a) Normal variation for both these enzymes is small. (b) Most of the variability is contributed by analytical variation. Within-individual variation is very small, this applying particularly to the male subjects' activity of alkaline phosphatase, and the aspartate transaminase activity of female subjects. (c) Elevation of the two enzyme activities above clinical normal ranges was infrequent, never of great magnitude, and occurred almost exclusively in males. When elevations of enzyme activity occurred, they tended to be associated either with particular males, or to occur on certain weeks. (d) When the latter effects were allowed for, and a new set of criteria calculated, the resulting ‘abnormal’ values now appeared to be more randomly distributed.