This paper describes the development and evaluation of a blended learning resource in the biosciences, created by combining online learning with formal face-face lectures and supported by formative assessments. In order to improve the effectiveness and efficiency of teaching large classes with mixed student cohorts, teaching was delivered through a variety of media which included three main components: (1) an interactive online tutorial, based on the cellular processes of DNA replication, damage and repair in relation to oncogenesis (2) formative assessment in the form of multiple choice questions to allow self evaluation and (3) small group follow-up workshops, to encourage deeper learning. The online tutorial was designed using Flash[C] software to help conceptualise complex cellular processes in time and space. It was supported by formative quizzes, references and printer-ready notes. Introduction of these resources in 2005 led to significant improvements in summative assessments across all student cohorts compared to scores from 2004. Students highly valued the usefulness of self-paced learning combined with supportive formative assessments which helped enhance the learning process. Teaching in small group workshops that followed on from the tutorials was also more effective, allowing a better interaction with the students, encouraging confidence and deeper learning among students. The efficiency of teaching was also improved with reduced assessment times and less pressure on institutional resources (availability of large lecture halls). This study therefore supports the use of blended learning as a means of improving both the effectiveness and efficiency of large group teaching. (Contains 1 figure and 1 table.)