The kinetics of monolayer cell cultures, derived from brain tumors which had been labeled with 3H-thymidine just prior to surgical removal, have been investigated. Analysis of autoradiographs indicates that: 1. the distribution of cell types in primary cultures is similar to that of the viable areas of the parent tumor in vivo; 2. the first week of culture constitutes a lag phase; and 3. vigorous proliferation commences during the second week of culture. Double labeling of the tritiated cultures with 14C-thymidine indicates that the pulse labeling index (PLI) of primary cultures during the second week is lower than that of the parent tumor and that the PLI of the culture increases during the second week. In contrast, we found that long-term established cultures in our laboratory have a high PLI during the first few days after passage, quickly reach a maximum, and then drop precipitously 1 week after passage. These latter variations in PLI correspond to the following growth pattern: 1. exponential growth; 2. a stationary phase due to medium exhaustion; or 3. a a plateau due to overcrowding. There are indications that some cell types, which have the capacity to divide in the parent tumor, lose this ability after transfer to tissue culture.