To the Editor.
—Thank you for asking for comments regarding the report of Kruit et al who failed to note a reaction in Swiss TNO mice in an attempt to duplicate our findings1 of an active agent contained in rheumatoid arthritis (RA) synovial tissue surgically removed from patients with RA.We have had difficulty in reestablishing a disease-free susceptible strain of Swiss Webster mice since moving our laboratory last July. Many of the newborn have a diffuse suffusion and dilated blood vessels and redness of one or more of the phalanges. In such cases, the typical RA reaction does not appear. Every female is now bred to assure the absences of these changes in her litter, for only in such animals do the typical reactions occur. Perhaps they have had this problem too.It often happens that source material of low potency will induce very mild reactions in mice