The biological potency of therapeutic allergen extracts is fundamentally important in terms of maximal security of immunotherapy, especially when changing batches. As regards pyridine-extracted alum-precipitated allergen preparations, the potency is commercially controlled by the RAST technique. In the present study, titrated intradermal tests were performed in thirty-three adult grass-pollen rhinitis patients with three ten-fold dilutions of two batches of Allpyral® 5-grass mix with different RAST activity. The difference in potency was estimated as the horizontal distance at the midpoint of the two dose–response curves after performing linear regression on the log-dose/log-weal area response, and as the concentration of each batch eliciting a weal equal to the histamine HCI 0.1 mg/ml reference. The difference in potency as estimated by RAST was a factor 5.0 vs a factor 8.3 and 7.2 (median) estimated by the horizontal distance and histamine-equivalent reaction, respectively. Considering the biological variation, the RAST-estimated difference in potency is in the same order as that found by skin test. It is. therefore, concluded that the routine control of new production batches by RAST is acceptable with the use of standardized reference sera as regards alum-precipitated grass pollen allergen extracts.