Through what mechanism do interest groups shape public opinion on concrete policies? In this article, three hypotheses are proposed that distinguish between the effect of the arguments conveyed by interest groups and the effect of interest groups as source cues. Two survey experiments on the proposed Transatlantic Trade and Investment Partnership (TIPP) and the 2015 Paris Agreement on climate change allow the testing of these hypotheses. The resulting evidence from several countries shows that, with respect to interest groups' attempts at shaping public opinion, arguments matter more than their sources. This is so even when accounting for people's trust in the interest groups that serve as source cues and for people's level of information about a policy. The finding that interest groups affect public opinion via arguments rather than as source cues has implications for the literature on elite influence on public opinion and the normative evaluation of interest group activities.