Using a standard analysis of one month's prescribing in a general practice we showed a potential saving of 8.8% (pounds 883.22) of the net cost of ingredients through substituting exact generic alternatives. Our results confirm the possible reduction in cost of drugs to the National Health Service by the use of generic substitution as suggested in the Greenfield report. Although substitution was theoretically available in most therapeutic classes, two thirds of the potential savings pertained to two classes, preparations acting on the nervous system and preparations acting on the cardiovascular system and diuretics. Of the prescriptions (31%) with potential generic alternatives, less than three quarters were actually available.