The fluorescent dye benzothiocarboxypurine (BCP) intensely stains nucleic acids. The dye does not penetrate viable white blood cells but does stain these cells following fixation. It has also been found that the dye stains the nucleic acid of viable Plasmodium falciparum. We have subsequently evaluated the staining of P. falciparum by benzothiocarboxypurine within red blood cells and have found that the red blood cell membrane is freely permeable to this dye and consequently P. falciparum is stained within the red blood cell. This finding prompted an in-depth analysis of the dye in the laboratory and in a field study as an alternative to Giemsa-stained blood smears and as a means of enhancing the microscopic diagnosis of malarial infection. In a field study the BCP dye allowed detection of malaria in fresh blood at a level equivalent to the Giemsa method (parasitemia ranged from 0.01% to 30%). The BCP staining procedure could also be used with fixed specimens although the differential staining characteristics were lost following specimen preparation. Of 111 blinded samples obtained in the field 22 were negative by Giemsa-stained thin smear, 16 were negative on thick smear and the same 16 were negative by BCP analysis. We have found that the BCP dye offers many advantages compared with the microscopic diagnosis of P. falciparum infection with standard Giemsa stains. These advantages are especially evident in conditions of low parasitemia, in the speed of staining and evaluation, and the relatively low level of training required to provide consistent results.