Providing fair assessment with timely feedback for students is a difficult task with science laboratory classes containing large numbers of students. Throughout our Faculty, such classes are assessed by short-answer questions (SAQs) centred on principles encountered in the laboratory. We have shown recently that computer-assisted assessment (CAA) has several advantages and is well received by students. However, student evaluation has shown that this system does not provide suitable feedback. We thus introduced peer assessment (PA) as a complementary procedure. In October 2006, 457 students registered for a first-year practical unit in the Faculty of Life Sciences, University of Manchester. This unit consists of ten compulsory biology practical classes. The first four practicals were assessed using PA; the remaining six practicals were assessed by CAA and marked by staff or postgraduate student demonstrators. The reliability and validity of PA were determined by comparing duplicate scripts and by staff moderation of selected scripts. Student opinions were sought via questionnaires. We show that both assessments are valid, reliable, easy to administer and are accepted by students. PA increases direct feedback to students, although the initial concerns of student groups such as mature and EU/International students need to be addressed using pre-PA training. (Contains 2 tables and 9 figures.)