The effects of N,N'-methylene-bis-acrylamide (MBA), a cross-linking agent, on blood and bone marrow after repeated oral doses, were studied in mice and rats. Body weight, three major elements of the blood - erythrocytes, leucocytes and platelets -, reticulocytes and bone marrow cells, were all reduced in either or both animals, especially in mice. Phenobarbital (PB) treatment did not greatly modify the effects of MBA in mice. An increase in free erythrocyte porphyrins and a decrease in ALA-D activity were observed in both animals. Urinary porphyrins were elevated in rats after MBA-dosing. PB-treatment did not significantly affect the elevation of porphyrins. After cessation of the MBA-dosing, all these changes were inclined to be restored to normal levels. Amounts of liver total porphyrins and microsomal P-450, and red cell fragility were within normal ranges in mice.