The magnetoresistance at temperatures below 20 K in an n-InSb/In(0.85)Al(0.15)Sb two-dimensional electron system is studied and described in terms of antilocalization due to quantum interference under strong spin-orbit interaction. The spin-orbit interaction coefficients are extracted by fitting the magnetoresistance data to an antilocalization theory distinguishing the Rashba and Dresselhaus contributions. A good agreement between magnetoresistance data and theory suggests a Rashba coefficient vertical bar alpha vertical bar approximate to 0.03 eV angstrom and a Dresselhaus coefficient gamma approximate to 490 eV angstrom(3). A strong contribution from the Dresselhaus term leads to pronounced anisotropy in the energy splitting induced by spin-orbit interaction in the two-dimensional electron dispersion.