Estimations of 6 serum enzyme activities were carried out in 519 subjects rigorously screened to exclude overt and latent disease, and obesity. The mean values of AP, GOT and GPT, and CPK were higher in males than in females. A negative correlation between age and activities of GPT and CPK was observed in males, whereas in females all activities measured, except that of GPT, showed a positive correlation with age. Only 5N and HBD failed to demonstrate a positive correlation with total body weight in one or other sex. A positive correlation between social class and AP was noted in females, and some significant correlations between diastolic blood pressure and some enzyme activities were found, but no association between enzyme activity and smoking habit could be detected. Computer programs were used to generate equations expressing enzyme activity as a function of age, total body weight, social class and diastolic blood pressure for each sex independently. From these, sets of reference values were derived which encompassed 95% of the demographically-corrected values for this healthy population. A trial of AP in medical out-patient clinics, and of 3 enzymes in cases of myocardial infarction, suggested that demographically-corrected reference values for serum enzymes may be more accurate than conventional parametric normal ranges in confirming or excluding organic disease.