Specific malaria and total IgM antibody responses were measured in 2 linguistically distinct coastal Papua New Guinean populations living in the same endemic malarious area, but exhibiting different adult female spleen rates (51% and 30%), in order to establish whether the higher spleen rates in the former group were due to hyper-reactive malarious splenomegaly (HMS). Malaria parasite rates were comparable, and geometric mean titres of IgG malaria antibody were the same, in both groups, indicating comparable exposure to malaria. A higher mean total IgM was observed in the high spleen (HS) rate group (6.07 g/litre, compared with 4.62 g/litre), a higher proportion was seropositive for IgM antibody to Plasmodium falciparum (63% compared with 54%), and HMS was found rather more frequently (4.7% compared with 2.6%). In both groups total IgM concentrations increased significantly with rising parity, and the mean level of 5.27 g/litre in young nulliparous women from the HS group suggested that IgM levels in this group at least were elevated from childhood. In both groups a rise in total IgM was associated with higher P. falciparum IgM geometric mean titres of antibody activity, a fall in parasite rates (HS group: 30% to 15%, P = 0.02; LS group: 24% to 0%, P = 0.034), and higher spleen rates (HS group: 38% to 65%, P = 0.001; LS group: 20% to 67%, P = 0.00012). It is concluded that the difference in spleen rates between the 2 groups was the result of differing degrees of acquired immunity to malaria, probably due to genetic differences in immune responses.(ABSTRACT TRUNCATED AT 250 WORDS)