Reported is the development, use, and evaluation of an instructional technique based upon: (1) the assessment of students' prior knowledge; and (2) a theoretical perspective advocated by Ausubel and others which emphasizes the importance of existing knowledge in influencing subsequent concept learning. The experimental group of 46 South African high school students was taught a series of concepts involving mass, volume and density using the special instructional strategy and materials. A control group (N=44) was taught the same concepts using a traditional strategy and materials. Comparison of pretest and posttest scores revealed significantly more knowledge gains for the experimental group. Implications of the results for instruction are discussed. (Author/WB)