Calmodulin, a calcium-dependent modulator protein, was shown to be indispensable for in vitro growth of erythrocytic stages of the human malaria parasite, Plasmodium falciparum. When the potent calmodulin antagonists, W7, trifluoperazine (TFP) and R24571, were added to cultures of P. falciparum they inhibited invasion of erythrocytes by merozoites, as well as maturation of schizonts. W5, a chlorine-deficient analogue of W7, was a much weaker inhibitor than W7. The concentrations of W5, W7, TFP and R24571 needed to produce 50% inhibition of schizont maturation were 63.5, 19, 18 and 8.5 microM, respectively, while concentrations needed to inhibit 50% the appearance of ring forms were only 19.5, 7, 8.4 and 4.5 microM, respectively. All the antagonists were more effective at inhibiting the invasion of erythrocytes by merozoites than maturation of schizonts. Ca2+ depletion by EGTA also inhibited merozoite invasion of erythrocytes. Unlike W5, W7, TFP and R24571, cyclosporin A (CsA) showed marked inhibition of schizont maturation at concentrations that reduce ring form production. Immunoelectron microscopy showed that calmodulin was concentrated at the apical end of both free and intraerythrocytic merozoites. No anticalmodulin immunoreactivity was observed in merozoites grown in the presence of 10 microM TFP, although the other calmodulin antagonists and EGTA did not significantly affect the calmodulin location in merozoites. These results suggest that the accumulation of calmodulin at the apical end of merozoites plays an important role during their attachment to and/or invasion of the host erythrocyte, possibly through activation of Ca2+ dependent processes.