In this paper we investigate the impact of leniency programs on firms’ decision to collude. We depart from previous literature by relaxing the assumption that evidence provided by a single firm suffices to convict an existing cartel with certainty. Assuming the conviction-probability to be increasing in the number of reporting firms, we show first that efficient cartel deterrence requires incentives for all firms to report. Under a regime that secures a marker for the first in line applicant, eligibility for leniency should be extended to at least a second informant. Further, we show that the introduction of the marker system has an ambiguous impact on cartel deterrence. In relation to the manner that the marker is secured and the cartel-related evidence is allocated, we derive the conditions under which allowing the first applicant to secure a marker enhances cartel deterrence.