Growth rate of the continuous murine alveolar macrophage single cell clones ZK1 (◆), ZK2 (■) and ZK6 (▲). Cells were seeded in six-well plate at 2 × 104 cells/ml and incubated at 37°C in a 5% CO2-humidified atmosphere in RPMI/10% FBS complete medium. Three wells per clone were harvested with cold PBS at 14 h and 18 h post seeding, and cells from each well were counted by a hemocytometer with trypan blue exclusion of dead cells. The obtained average cell count for each clone at each time point was plotted against the time. Doubling time (mean generation time = mgt) was calculated according the formula: N = N02T/mgt. N is the number of cells at any time T; N0, is the number of cells at an initial point. The doubling time of ZK cell lines is approximately 14 hours.