A procedure has been developed for the accurate, quick, and simple indexing of the growth of insect cell monolayers with a reticule in the eyepiece of an inverted phasecontrast microscope. A magnification of ×500 is desirable for accuracy and ease of counting. Cell nuclei whose circumferences include any of the 25 points in the reticule are enumerated in each microscopic field. Counts of as few as five fields gave an error of 18.8% per flask. Ten fields gave an error of 11.5% and provided a sufficiently accurate comparison for many purposes. Forty counts allowed the measurement of differences between treatments with an error of 5.85%. A large number of treatments can be handled simultaneously, due to the small number of replicates necessary and the small amount of time required per treatment. When the capacity of different batches of lactalbumin hydrolysate to support growth was tested in our insect tissue culture medium, some of them were found to be suitable and others unsuitable. Doubling time of cell populations was measured, after the ratio of the average area of the nuclei at time 0 to that at timeX was used to multiply the number of nuclei counted at timeX. Average nuclear areas were satisfactorily measured by simple measurements of nuclear diameter on an ocular micrometer. The cell nuclei tended to decrease in area when cell monolayers reached confluence or became crowded. The number of replicates required was reduced, because the same flask could be used several times without disturbing the cell monolayer. The method of counting nuclei in a monolayer by means of reticule points and a phase-contrast microscope can also be adapted to the estimation of the absolute number of cells growing on the bottom of a flask.