Quantitative data is presented concerning the influence that age, sex, the menopause, oral contraception, blood group and time of blood collection have on 19 commonly-determined blood chemistry values. The data were derived from 1000 healthy blood donors in whom blood sampling conditions were standardised to conform with those applying to hospital out-patients. Statistical techniques were used to allow for the effects of analytical variation and to enable the effects of the various factors and their interactions to be expressed in a practically useful manner. Age and sex effects are shown to be the rule and to interact in many instances. Creatinine, urea, glucose, cholesterol, potassium and globulin show a tendency to increase in concentration with age, while total protein, albumin, calcium, inorganic phosphate and iron levels tend to fall progressively. Evidence is given to suggest that many of the changes occurring in women at about 50 years should be attributed to hormonal changes. In all instances when the menopause has a significant influence there is a rise in the concentration of the constituent concerned. It is proposed that other laboratories should assemble age and sex stratified reference ranges from (a) figures given for the mean values for defined age and sex groups, adjusted if necessary to allow for different analytical bias, (b) figures given for nonsystematic biological variation, and (c) their own measurements of analytical precision. Details of the necessary procedure are given, allowance being made for logarithmic transformation of the results where necessary.