To test the efficacy of chlorproguanil prophylaxis, 156 malaria-free schoolchildren in the coastal region of Kenya were allocated at random to receive either 7.5 mg chlorproguanil daily, 50 mg chlorproguanil weekly, 100 mg proguanil daily, or 100 mg calcium lactate weekly (placebo). The children were followed up daily for 169 d, by which time Plasmodium falciparum parasitaemia had occurred in 92% of the placebo group, 31% of the daily proguanil group, 38% of the daily chlorproguanil group and 55% of the weekly chlorproguanil group. There was significant reduction (P < 0.001) in the risk of parasitaemia in all the groups receiving chemoprophylaxis. Daily chlorproguanil and daily proguanil were equally effective, and significantly more effective than weekly high dose chlorproguanil. No significant toxicity was reported or observed. Thus daily chlorproguanil 20 mg/60 kg is a cheap and effective alternative to proguanil for chemoprophylaxis.