A sensitive fluorometric method for assaying malarial pigment, haemozoin, has been developed and used to determine the haemozoin content of blood and tissue samples. Plasmodium falciparum rings and trophozoites were found to contain 23 and 339 ng haemozoin/106 parasitized red blood cells (PRBCs), respectively. Unsynchronized Plasmodium berghei NK65 or ANKA parasites from infected mice contained 27 and 61 ng haemozoin/106 PRBCs, respectively. An exponential accumulation of haemozoin within 18 days after infection was demonstrated in liver and spleen tissue, representing up to 0·2% of the tissue by wet weight by day 18. Histology indicated that the accumulation occurred predominantly in the tissue monocytes. In the brain, the levels of haemozoin after 8 days of infection were considerably lower than they were in the liver or spleen, and most of the pigment appeared to be that present inside parasitized red blood cells. CBA/Ca mice infected with P. berghei ANKA (a cerebral malaria model) had significantly higher amounts of haemozoin in the brain than did ICR mice infected with P. berghei NK65. Thus, haemozoin levels in tissue increase with the duration of infection, and its presence may be associated with cerebral pathology.