Isoenzymes were 'discovered' 20 years ago and were at first regarded as interesting but rare occurrences. Since then a wealth of information on enzyme heterogeneity has accrued and it now seems likely that at least half of all enzymes exist as isoenzymes. This is important in many areas of biological and medical science. Thus isoenzyme studies have provided the main experimental substance for the neutral drift controversy in genetics and evolution; they have greatly extended our understanding of metabolic regulation not only in animals but also in bacteria and plants; their existence has made...