  We report results of soft gamma-ray observations of the ultraluminous
infrared galaxies Arp 220, Mrk 273, and Mrk 231 in order to test whether the
infrared radiation from these sources originates from buried active galactic
nuclei (AGNs). Only upper limits are measured, implying that the emergent soft
gamma-ray luminosities are 1-2 orders of magnitude smaller than the infrared
luminosities. Monte Carlo simulations of radiation transport through tori are
used to infer the minimum column densities $N_H$ required to block transmission
of soft gamma-rays from a buried AGN, assuming that spectra of AGNs in such
sources are similar to those of radio-quiet quasars. Lack of measured gamma-ray
emission provides no supporting evidence for the existence of buried AGNs in
these galaxies, but is consistent with an origin of the infrared luminosity
from starburst activity.
