We compute the mean attenuation length (&lgr;f) in Earth's atmosphere for the protons produced in the solar flare of 29 September 1989. We use data obtained with three pairs of neutron monitors. The detectors in the pair have very similar values of the geomagnetic vertical cut-off rigidity (R0) and overlapping asymptotic cones of acceptance but they differ in terms of the altitudes of the observing sites. We apply Wilson et al. [1967> method to compute the values of &lgr;f for the three representative pairs which cover a range of solar proton rigidities. The computed mean values of &lgr;f are significantly larger than the consensus value of (100¿5) g cm-2 reported in the literature [Duggal, 1979>. Also, they exhibit a linear correlation with R0. ¿ American Geophysical Union 1993