Chromosomal analyses and life span studies of WI-38 cell cultures used for vaccine production at the Connaught Laboratories Ltd will be described. Twenty cultures derived from the same seed lot and 25 cultures derived from different starter cultures were analyzed. Karyology tests at population doublings 31 and higher were within acceptable limits. The WI-38 cells reached a finite lifespan of approximately 65 population doublings. Normal growth characteristics were observed within 2/3 of the finite lifespan, i.e. until population doubling 40. The finite lifespan of MRC-5 was found to be 75 population doublings, with normal growth pattern until passage 50. The karyology test results, cell yields and the finite lifespans indicated that these cells could be used as substrates for vaccine production at higher passage levels than previously suggested. Using serially subcultivated cells at higher passage levels would make possible an increase in the volume of vaccine from existing cell seed and would also result in significant reduction in production costs.