Expression of the CD45RO putative memory cell antigen on CD4 (helper) and CD8 (cytotoxic/suppressor) lymphocytes of children born to HIV-infected women was investigated using the UCHL 1 antibody. Significantly raised numbers of CD45RO + CD8 lymphocytes were found in all nine of the infected children compared with uninfected and control children. Expression of CD45RO on CD4 lymphocytes was variable; absolute numbers were not increased, although the percentage was increased in four out of nine infected children. All the infected children except two (who had comparatively low numbers of CD45RO+ CD8 cells) were clinically well, which suggests that an increase in CD45RO + CD8 cells may be indicative of a functionally active immune response against HIV.