To summarize, absorbance measurements can be very useful but must be used with discretion. We use them in almost every experiment for guiding the experiment as it is in progress. We use them as data when a highly accurate measurement of cell concentration does not seem to be required We use them, with precautions, to determine growth rates. A procedure that avoids some uncertainties is to obtain a population in exponential growth at a rather low cell concentration, todetermine theabsorbance,and then to dilute the culture and time the return to the original absorbanc seems likely that the relation between absorbance and ce l number is the same each time the population is at that particular absorbance value, a that a valid generation time therefore results. We do not think that a bance measurements should substitute for direct measurements of more basic parameters, such as cell number and dry weight, when accuracy is desired, and we do not think that absorbance values should ever be re- ported without first being converted properly to values of one of the more basic parameters