Studies related to the measurement of growth inhibition of HeLa cells by 6-Thioguanine (TG) and Vermiculine (V) have been described. Control and treated cell populations were enumerated as uniform suspensions of whole cells, prepared by standard treatment with trypsin. Determination of cell number, measurements of protein, DNA and RNA as well as of glucose consumption from the culture medium were made throughout the treatment with the studied drugs. It was apparent that the total cell number in control cultures was directly proportional to the total cell protein or nucleic acids. However, in both treated cultures the relationship between these parameters was disturbed as consequences to unbalanced growth which was an integral part of the cytotoxic reaction of the studied agents. From the results presented it followed that only the direct cell enumeration was suitable for the detection of agents' cytotoxicity.