A series of C-3 thiourea functionalized β-lactams, β-lactam-7-chloroquinoline conjugates and 7-chloroquinoline-thiohydantoin derivatives were prepared with the aim of probing antimalarial structure-activity relationships. 7-Chlorquinoline-thiohydantoin derivatives were found to be potent inhibitors of cultured Plasmodium falciparum, with the most potent and non-cytotoxic compound exhibiting an IC50 of 39.8 nM. Studies of β-hematin formation suggested that inhibition of haemozoin formation could be primary mechanism of action, with IC50 values comparable to those of chloroquine. Evaluation of cytotoxicity against HeLa cells demonstrated high selective indices.