The heterogeneity of pyruvate kinase (PK) deficiency associated with hereditary haemolytic anaemia is emphasized by studies of a kindred harbouring two distinct mutant forms of this enzyme, both of which were kinetically defective with markedly decreased affinities for the substrate, phosphoenolypyruvate. The two isoenzymes, designated PK-Vancouver1 and PK-Vancouver2, were primarily distinguishable from one another by differences in maximum in vitro activities and by variations in response to fructose-I,6-diphosphate activation. When combined in proband erythrocytes to the exclusion of any normal PK, the isoenzymes were associated with a severe chronic haemolytic process with many of the features of PK deficiency of the more common quantitative type. Clinical laboratory screening tests for detecting PK deficiency may be falsely negative or equivocal in such cases.