Several experiments were carried out to measure the ability of neonatally thymectomized Lewis rats (NTLR) to limit multiplication of Mycobacterium leprae. NTLR inoculated in one hind footpad with 10(7) viable M. leprae and challenged in the other hind footpad with 5 x 10(3) organisms simultaneously or 120 or 180 days later permitted multiplication in both sites. By contrast, immunologically intact rats similarly inoculated did not permit multiplication from either inoculum. NTLR and immunologically normal BALB/c mice were equally susceptible to infection with M. leprae, in that multiplication occurred regularly in the footpads of both species when inoculated with a bacterial suspension diluted to provide five organisms per footpad. Finally, multiplication occurred when five viable M. leprae diluted with 10(7) heat-killed organisms were inoculated into the footpads of NTLR. Although there was some evidence that NTLR are not completely immunosuppressed, NTLR appear to be capable of detecting much smaller proportions of viable M. leprae than can be detected by immunologically normal mice.