Activation of peritoneal macrophages from guinea pigs by various bacterial cell walls, M-1 endo-N-acetylmuramidase enzymatically digested bacterial cell walls and synthetic muramyl dipeptides was studied in terms of stimulation of [14C] glucosamine incorporation. All test bacterial cell wall preparations significantly increased a [14C]glucosamine uptake by the macrophages. Some of the water-soluble M-1 enzyme digests also exerted stimulating effects on macrophages, although the activity of the digests was found to be weaker than those of original cell walls. Furthermore, an adjuvant-active synthetic MurNAc-L-Ala-D-isoGln (MDP) showed a weak but significant activity, whereas an adjuvant-inactive analog, MurNAc-L-Ala-L-iso-Gln, did not show a significant activity, at least with the dose of 100 microgram. Additional studies with 6-O-acyl derivatives of MDP revealed that 6-O-(2-tetradecylhexadecanoyl)-MDP and 6-O-(3-hydroxy-2-tetradecyl-octadecanoyl)-MDP exhibit stronger macrophage-stimulating effects than MDP. It can be concluded from the above findings that MDP is the essential structure responsible for stimulating the activity of cell walls on guinea pig peritoneal macrophages, but it requires a particle state, which results from an additive character of lipophilicity, to exert the activity fully and effectively.