IgA, IgM and IgG concentrations and their bacterial antibodies to E. coli, group B streptococci and Brucella abortus were measured in human breast milk collected from the 1st to 10th day post-partum from mothers delivered of 'preterm' infants (Premature Breast Milk or PBM) and from mothers delivered of term infants (Term Breast Milk or TBM). Reverse passive haemagglutination tests (RPH), rocket immuno-electrophoresis and mixed reverse passive antiglobulin haemagglutination tests (MRPAH) were employed. PBM at 2-5 days post-partum (though not beyond this period) contained higher IgA levels than did TBM, and this difference persisted even when total IgA was expressed as a proportion of total milk protein: in contrast the IgM and IgG contents of PBM and TBM were the same at both these postnatal ages. The titre of IgA antibody to E. coli, which was absorbable only by the corresponding bacteria, showed no significant difference between PBM and TBM, whereas the titres of IgA reacting with Br. abortus and, to a lesser extent group B streptococci, were higher in PBM than those in TBM. However the IgA which reacted with Br. abortus and group B streptococci was not specific to those organisms but was absorbed by all three bacteria studied. It is speculated that the high IgA content of early preterm milk and perhaps the presence of especially high titres of what appears to be a non-specific or cross-reacting bacterial IgA in such milk, may be immunologically advantageous to low birthweight infants fed on their own mother's milk.