When NCTC clone 929 (strain L) cells were grown either continuously or at intervals in toxic human serum, they became resistant to the toxic substance or substances. The resistance developed to toxic human serum was passed from one cell generation to another; i.e., it was heritable. The resistance to toxic human serum, developed in strain L cells, was associated with a decrease in their ability to adsorb toxic substances from the serum. This suggests that there is a decrease in the number of loci in the resistant cells where toxic substances can be bound. The resistant cells differed from the sensitive cells in their morphological appearance, the increased frequency of minute chromosomes, and the increased adhesion to glass surfaces. The cells which developed a resistance to a particular toxic serum also resisted the toxic effects of some other sera, but not of all.