To determine the effects of chloroquine phosphate (300 mg base weekly) chemoprophylaxis for malaria provided by volunteer village health helpers (VHH's), pregnant women attending antenatal clinics in Saradidi, Kenya, were examined each month. Parasitaemia, haemoglobin level, and the presence of urinary 4-aminoquinolines were determined at each visit. The age composition and parity of women taking chemoprophylaxis were not statistically significantly different from those of the other women. A total of 104 (29.1%) of 357 pregnant women from 23 villages where chemprophylaxis was provided by VHH's said they were taking it. Women 30 to 44 years of age (43.9%) of 82) were more often taking prophylaxis than those younger (25.1% of 271) (P less than 0.0005). An additional 573 pregnant women to whom regular chemoprophylaxis was not provided from 33 control villages were also examined at least once. When compared with those from women not taking prophylaxis, blood samples from pregnant women on antimalarial prophylaxis had lower parasite rates (17.7% of 265 compared with 26.2% of 1700, P less than 0.005), higher haemoglobin levels (59.1% of 127 were greater than or equal to 10.0 g l-1 compared with 49.7% of 1111, P less than 0.05), and a higher mean haemoglobin level (9.95 g dl-1 compared with 9.62, P = 0.019) and urine samples were more often positive for 4-aminoquinolines (15.7% of 255 compared with 8.3% of 1656, P less than 0.0005). For women with two or more parasitologic samples, 69.6% of 79 pregnant women on prophylaxis had no parasites found on any visit compared with 51.6% of 516 women not on chemoprophylaxis (P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)