Giemsa-stained smears of T4-treated and untreated cultures. T4 was added at 40 nM at the early ring stage. The morphology of parasites in treated and untreated cultures is shown (early trophozoites, T0+24 h; late trophozoites, T0+30 h; schizonts, T0+36 h; early ring stage, T0+43 h; late ring stage, T0+48 h; and trophozoites, T0+71 h). Morphological changes are clearly observed at 36 hours with absence of schizont formation (T0+36 h). Between 43 and 48 hours surviving parasites are blocked at the trophozoite stages. At 71 hours parasitemia falls to negligible level.