The fate of 3H-nor-muramyl dipeptide has been investigated after administration with bovine serum albumin (BSA) in Freund's incomplete adjuvant in the guinea pig footpad. Emulsions of two different stiffnesses , both of which were capable of inducing delayed-type hypersensitivity to BSA, were compared. Both emulsions produced stable depots of intact nor-MDP at the injection site. At early times retention of nor-MDP was greater with the stiffer emulsion and circulating levels of nor-MDP were greater with the looser emulsion; at 24 h there were no differences between the two. Distribution of nor-MDP to the draining lymph nodes was highly variable and no difference was apparent between the two emulsions. Levels of radioactivity in more distant lymph nodes were minimal, and in other tissues no radioactivity was detected. Thus it has not been possible to clarify the role of nor-MDP in this system in terms of its distribution through the local lymphatics, however, some information has emerged about the stability and integrity of nor-MDP in local adjuvant formulation.