The 1-6 ^m channel of the Along Track Scanning Radiometer-1 (ATSR-1) does not benefit from a pre-launch or onboard digital count to spectral radiance calibration mechanism. This Letter outlines the results of a post-launch calibration procedure, accomplished by comparing near-coincident data from the uncalibrated ATSR-1 instrument onboard ERS-1 and the (calibrated) OPS instrument onboard JERS-1. These results show that the ATSR-1 1-6(im channel can be considered linear in response to incoming radiance and equations are produced that allow data from this channel to be calibrated in units of spectral radiance and/ or spectral reflectance.