This report, one of a series on research into specific physical activities and their efficacy in improving and maintaining physical fitness, examines sport participation and the potential it has for developing muscular strength, muscular endurance, and circulatory-respiratory endurance. The activities consist primarily of the following twelve sports: archery, badminton, basketball, bowling, handball, field hockey, ice hockey, paddleball, skiing, soccer, tennis, and volleyball. Documentation is presented rating these sports into groups of "vigorous" and "mild" activities. The contribution that participation in milder activities makes to the improvement of physical fitness is examined. Three comprehensive evaluations of contributions from participation in various activities to the improvement of physical fitness components are made, one evaluation drawn from the opinions of physicians and two from exercise systems proposed in research. A summary analysis of the physical fitness benefits realized from participation in each of the twelve sports is presented. Three basic principles to which participants in physical activity should pay heed are defined: (a) exercise should be adapted to the individual's exercise tolerance; (b) reasonable "overloading" should be applied to induce higher levels of performance; and (c) the exercise should be progressive. (MJB)