Summary The immunologic function of lymph node cell transplants between guinea pigs of a highly inbred, histocompatible strain (isologous transfer between Strain 13 guinea pigs) was compared to the function of transplants to incompatible recipients of another strain (homologous transfer: Strain 13 to Hartley guinea pigs). Lymph node cell suspensions were prepared during the second week after immunization of donors with complete Freund’s adjuvant. The functional persistence of intraperitoneally injected donor cells was assessed by the degree of delayed cutaneous hypersensitivity of cell recipients to purified protein derivative (PPD). The transfer of immune cells in quantities inadequate to confer significant cutaneous hypersensitivity during the first posttransfer week, resulted in a high degree of tuberculin sensitivity in isologous recipients during the second and third weeks. In contrast, homologous recipients manifested no significant cutaneous reactivity during the 4-week period following identical cell transfers. The data are compatible with the interpretation that isologous recipients manifest adoptive immunity (passive immunity) which depends on the continued function and proliferation of surviving immune cells from compatible donors. The observation that the cells of one donor can confer a degree of cutaneous hypersensitivity as high as that seen in actively immunized (Strain 13) guinea pigs, may be related to graft-host compatibility as well as to the use of donors during the phase of rapidly ascending immunization and hyperplastic proliferation of lymph node cells. In isologous recipients, the degree of cutaneous reactivity in terms of the surface area (r2) of skin reactions was proportional to the number of viable cells transferred. This relationship was not observed in homologous recipients. Freeze-killed and sonically disrupted lymph node cells were either ineffective or minimally effective in conferring cutaneous tuberculin sensitivity. Spleen cells were found similarly ineffective in transferring tuberculin sensitivity. Repeated skin testing with PPD induced a detectable level of cutaneous sensitivity in normal guinea pigs. This sensitization appears enhanced in lymph node cell recipients.