Experimental work was carried out to establish the growth characteristics of Plasmodium falciparum in an in vitro culture system using cells with the Dantu, Henshaw and S-s-U- blood-group variants. A flow cytometric technique, using the dye thiazole orange, was adapted for use on the Epics Profile II flow cytometer to count the parasites. This was performed at 24, 48 and 72 h. The ability of the parasites to grow in red cells of the Dantu and Henshaw phenotypes was also assayed by 3[H] hypoxanthine incorporation. Relative to control red cells, S-s-U- cells and Dantu cells were less suitable as host cells for P. falciparum in vitro. In contrast, cells expressing the Henshaw antigen were equally sensitive to P. falciparum infection as were normal controls. These data support the notion that glycophorins play an important role in P. falciparum infection. Further studies are required to evaluate the epidemiological significance of these results.