We have observed the double-spin-flip mode, corresponding to one photon flipping two nuclear spins, in rhodium with low-frequency SQUID NMR at ultralow temperatures. The shift and magnitude of the absorption line at around twice the Larmor frequency has been studied. The shift of the double-spin-flip mode can be used to calculate the relative strength of the Ruderman-Kittel exchange interaction, for which we obtain R = - 0.9 +/- 0.1. (C) 2000 Elsevier Science B.V. All rights reserved.