Thirty-eight mothers and babies were assessed by the lymphocyte transformation test for their cell-mediated immune responses to Treponema pallidum and to purified protein derivative of tuberculin (PPD). All babies, including controls, appeared to be reactive when results of [3H]-thymidine incorporation were expressed as net counts per minute. This was because of high background activity, which was most marked in cells from neonates. When results were expressed as stimulation ratios, it was revealed that cells from neonates were unreactive to both antigens, even when those from their mothers were reactive. Cells from some older babies with active congenital syphilis were reactive to both antigens.