Studies of school-based mouthrinsing programs show variations in costs. In earlier work per subject costs in 14 programs were shown to increase with program size. Theoretical explanations for this finding, pertinent to these programs, include 1) wage differentials and 2) scale effects--differences in labor productivity attributable to organizational differences. To test the hypotheses implicit in these explanations a log-linear model was specified with average costs (in constant dollars) expressed as a function of wages and scale. Data were pooled for two complete years, yielding 28 observations. Regression analysis showed that 68% of the variation in average costs was explained by wages and scale, and that with wages held constant, a 1.0% increase in program size correlated with a 0.33% increase in per subject costs. The result is attributable to decreasing labor productivity. The findings of diseconomies of scale in mouthrinse programs have potential implications for the planning and evaluation of community dental programs.