Rapidity of thymic and splenic reconstitution in SCID recipients treated via IUT. BALB/c-SCID fetal recipients received 10⁶ B6 TCD BM cells on day 15 to 16 of gestation. Controls consisted of noninjected BALB/c-SCID and noninjected BALB/c wild-type mice. At the indicated times postnatally, 4 to 8 mice were studied for thymic (A) and splenic (B) cellularity. *Significant differences in cellularity as compared with normal age-matched B6 controls. The absolute cell number is listed on the y-axis. The bars designate one standard error of the mean.