We report on the in vitro effect of a thymic factor (TF) extracted from pig thymuses, on human lymphoid cells from umbilical cord blood and from peripheral blood of 8 T cell-deficient patients. E rosette formation was not affected by TF when tested on cells from peripheral blood of normal adults. With cells from umbilical cord blood of 13 healthy, full-therm newborn babies, the difference between the percent (mean) of ERFCs before (16.31 ± 11.13) and after (28.85 ± 17.10) incubation with TF was statistically significant (p <0.05). In most samples, TF transformed about 10–20% of the cells. In the T cell-deficient group the increase in ERFCs of the peripheral blood lymphocytes, though consistent, was variable in degree from case to case. Our data indicate that precursor cells in some individuals with T cell deficiency are very sensitive to TF. Patients with highly responsive precursors appear to be the best candidates for a therapeutic approach with TF when thymus transplant is not possible.