The aim of the study was to test the hypotheses (i) that sudden infant death syndrome sera are toxic to 11-day old chick embryos and (ii) that such a toxicity can be counteracted by immunoglobulin or adult sera. Serum samples from 11 SIDS victims and five controls were tested for lethal toxicity in the chick embryo bioassay. Five serum samples were used to challenge chick embryos injected with the following: sudden infant death syndrome serum plus Hank's balanced salt solution; Hank's balanced salt solution alone; sudden infant death syndrome serum plus 3% w/v commercial immunoglobulin; sudden infant death syndrome serum plus 6% w/v immunoglobulin; sudden infant death syndrome serum plus pooled sera of 40 healthy adults. Results obtained revealed that Hank's balanced salt solution, the pooled adult serum and the commercial immunoglobulin were all non-lethal, in the chick embryo test system. By contrast. 10 sudden infant death syndrome victims yielded sera containing lethal levels of toxin(s) compared to 2/5 controls which was statistically significant (P < 0.05, Fischer's exact test). In the tests of sudden infant death syndrome serum plus immunoglobulin or pooled adult serum, the lethality of sudden infant death syndrome serum was abolished in all cases. The reduction in toxicity of individual sudden infant death syndrome serum plus immunoglobulin or pooled adult serum was often statistically significant (P<0.05-P<0.00005, Fischer's exact test). We conclude that lethal levels of toxin are present in sudden infant death syndrome sera and that they can be neutralised by normal immune serum. These results indicate that passive immunisation is a potential treatment to protect babies considered at risk from sudden infant death syndrome.