School-based assessment of science students' practical skills has two important roles--as a complement to written papers in public examinations and as a catalyst for enriching the science curriculum in schools. This article describes a quantitative study of the concerns chemistry and biology teachers experience as they engage in the process of implementation of a school-based assessment scheme for practical work. A 23-item questionnaire was developed to measure five categories of teacher concern: evaluation, information, management, consequence and refocusing. The nature of each category of teacher concern is discussed in relation to innovation adoption and implementation. Data were collected from 400 chemistry and 412 biology teachers in Hong Kong. Teachers' information and management concerns lessened in intensity when they became experienced users of a school-based assessment scheme. However, teaching experience alone could not motivate teachers to think more about the impact of school-based assessment on student learning, their professional development in student assessment and the possible refinements in their school-based assessment scheme. Concerns-based interventions are suggested to help teachers grow professionally.