The present invention relates to two novel crystal forms of a known compound, 1-[5-Methanesulfonamidoindolyl-2-carbonyl]-4-[3-(1-methylethylamino)-2-pyr idinyl] piperazine monomethanesulfonate salt, which are useful for treating humans who are HIV positive, which are identified by a powder X-ray diffraction spectrum known commonly as the "S" and "T" forms.