In 20 neomycin-allergic patients as well as 50 normal control volunteers, 48-hour patch tests were applied in varying dilutions to neomycin, gentamicin, and a new aminoglycoside antibiotic, butirosin sulfate (ambutyrosin; (CI-642). Also, intradermal tests to all antibiotics were performed on all patients as well as control volunteers. From this study we conclude the following: (1) Fase-positive patch tests or intradermal responses did not occur in 50 control volunteers to any of the antibiotics in any of the concentrations used. (2) In the 20 neomycin-allergic patients cross-sensitization exists with gentamicin (55%) and butirosin (90%). (3) Butirosin should not be used to treat neomycin-allergic patients when it becomes commercially available.