The transfer factor (TLCOsb) is currently widely used as a lung function test. Although the test maneuver itself is well described and uniformly approached by most workers, the computation technique varies considerably. Significant changes in the TLCOsb are induced by correcting for: blood hemoglobin concentration [Hb], apparatus and anatomic dead space (VD), alveolar gas sample dead space (VDbag), carboxyhemoglobin concentration (COHb), alveolar carbon dioxide fraction (FACO2), initial inspired gas (VI) conditions, and breath-hold time (t). The quantitative impact upon TLCOsb of the presence or absence of corrections was calculated using TLCOsb measurements from 245 normal subjects. The average change (%) in computed TLCOsb induced by correcting for the above variables is: Hb (+8%); VD (-8%); VDbag (-0.3 to -6%); COHb (+X% for X% COHb); FACO2 (+5%); VI (+4%); t (+7%). Since corrections are made by some laboratories and not by others and since no uniformity exists concerning the corrections to be made, it is possible for two laboratories to choose their computation routines in such a way that they would compute, from the same test results, TLCOsb values which differ by 41%. Standardization of the TLCOsb computation technique is needed.