A 90-dose LD50 (or ED50) and a chronicity factor are proposed to help communicate the results of tests involving repeated doses of compounds. To determine the oral 90-dose LD50, groups of animals are fed appropriate dietary levels of a compound for 90 days and then held long enough for any sick survivors to die or recover. Food consumption is recorded and doses are expressed as milligrams of compound per kilogram of body weight per day. Statistically, the 90-dose LD50 is determined in the same way as the 1-dose LD50 using logarithms of doses and percent mortality expressed as probits. A ratio of the 1-dose LD50 and 90-dose LD50 of a compound is a measure of its cumulative effects and is termed the chronicity factor. The largest one found so far is more than 500 times the smallest. These factors permit objective comparison of different classes of compounds, but whether the smaller distinctions within a class are significnat, remains to be learned.