Eighteen infants born to anti-HIV-positive mothers were tested bimonthly for immunoglobulin M (IgM) anti-HIV by Western blot and HIV p24 antigen (Ag) by enzyme-linked immunosorbent assay (ELISA) in order to determine the role of these markers in the early diagnosis of HIV infection. Twelve healthy infants were also studied as a control group. In 11 out of 18 children (61.1%) an IgM response was demonstrable, in 13 out of 18 (72.2%) IgM anti-HIV and/or p24 antigen (Ag) were detected. Two patterns of IgM response were identified: a precocious IgM positivity (group of five children positive at birth) and a later appearance of IgM, always within the third month (six cases). Early p24 antigenemia occurred in one infant. Three out of four children who developed antigenemia after birth were symptomatic within the sixth month. No clinical or immunological abnormalities were found among the three children who were persistently negative for both IgM anti-HIV and p24 Ag. Serial IgM anti-HIV and p24 Ag testing may be helpful in the early identification of HIV-infected patients.