This study tested a causal model of the influence of tutor behavior on student achievement and interest in the context of problem-based learning. Data were gathered from 524 tutorial groups involving students in the health sciences curriculum at the University of Limburg in the Netherlands during 1992-93. Correlations among the 261 tutors' social congruence, expertise use and cognitive congruence behaviors, and small-group functioning and students' self-study time, intrinsic interest in the subject matter, and level of achievement were analyzed using structural equations modelling. The study found that the tutors' level of expertise use and social congruence not only directly affected their level of cognitive congruence but also affected other elements of the model. The level of cognitive congruence influenced tutorial group functioning, which in turn affected student self-study time and intrinsic interest. The results suggest that subject-matter expertise, a commitment to the students' learning and their lives in a personal, authtentic way, and the ability to express oneself in the language used by the students are all determinants of learning in problem-based curricula. (Contains 14 references.) (MDM)