Researchers are skeptical about the role of authoritative sources of information in a constructivist learning environment for fear of usurping students' critical thinking. Taking a social semiotics perspective in this study, authoritative sources are regarded as inscriptions of cultural artifacts, and science learning involves meaning-making of these cultural artifacts. In studying the meaning-making process of a group of students doing problem-based learning (PBL), our findings show that authoritative sources played an important role in deepening and expanding students' scientific knowledge. We also found that constructive use of authoritative sources involves interpretation of meaning in context. This structural coupling of authoritative sources and context for meaningful sense-making has to be taken into consideration in the design of learning environment. (Contains 3 figures.)