A method for measuring erythrocyte transketolase activity (ETKA) is described that is precise, economical of reagents and capable of high throughput with partial or full automation. The transketolase-dependent transformation of ribose-5-phosphate, yielding glyceraldehyde-3-phosphate, is linked via 'indicator enzymes' to glycerol production and NADH consumption, the latter being followed fluorimetrically. Conditions under which ETKA and 'TPP Effect' are derived have been examined and optimised. Values are comparable with those obtained by other methods but a relatively narrow adult reference range for ETKA is observed. Data are presented for optimal preparation and storage of samples.