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Hypotheses formulated from a previous study in Greece concerning the relationship between responses to hepatitis B virus (HBV) infection in parents and the secondary sex ratio of their offspring were tested in the population of Kar Kar Island, Papua New Guinea. The results were similar to those observed in the Greek village population. Sex ratios were highest (115) when either parent was a chronic carrier of HBV (HBsAg(+)) and lowest (60) when both parents were HBsAg(—) and the mother had antibody to HBsAg (anti-HBs(+)). Couples in which both parents were antigen and antibody negative had offspring with sex ratios intermediate (84) between the antigen positive and mother antibody positive couples. As in Greece, the sex ratio effects were due mainly to differences in the number of daughters, not sons, in each family; either parent antigen positive couples had the fewest (2.3) and mother anti-HBs(+), father HBsAg(—) couples had the most (3.9) daughters per couple. As a consequence, the latter couples tended to have larger total family sizes. Possible mechanisms to explain these observations are discussed.