The objective of the report is: (1) to compare alternative ozone exposure indices that relate well to plant response and are useful in developing future standards for ozone; (2) to estimate standard errors and confidence intervals of regression coefficients and exposure levels of fixed yield losses. Plant growth data for soybean, wheat, corn, sorghum, and cotton, from NCLAN studies were regressed against the exposure indices. Optimum performance, when the minimum sum of squares criterion was used, was not attained for any single index across crops/species, sites, and years. However, indices that cumulated concentrations over time and emphasized ozone concentrations of 0.06 ppm or higher correlated well with plant growth. The censored cumulative sum of ozone concentrations of 0.06-0.07 ppm and a sigmoid-weighted cumulative index (centered at 0.062 ppm) were among the topranked indices, having potential as a future standard. These findings indicate that: (1) peak concentrations are important but lower concentrations should be included in the calculations; (2) increased plant sensitivity occurs between flowering and maturity; (3) plants respond to cumulative impact.