In the late 1930s, Grigulevich was sent to Spain to monitor the activities of the Workers' Party of Marxist Unification (POUM, the militia with which George Orwell served), during the course of the civil war in that country. Grigulevich worked under NKVD general Alexander Orlov, using the code names MAKS and FELIPE, and organized so-called "mobile groups" that killed, among other actual and suspected Trotskyists, POUM leader Andrés Nin. In this mission Grigulevich apparently collaborated with the assassin Vittorio Vidali, known in Spain as "Comandante Carlos Contreras."