Blood histamine levels (BHL; ng histamine base/ml blood, mean +/- SEM) were determined in 118 infants born to HIV-1-infected mothers and in children infected after blood transfusion. In asymptomatic infected infants, BHL were not significantly different from HIV-1-negative infants born to HIV-infected mothers (54.1 +/- 5.6 vs. 56.4 +/- 3.5). BHL progressively decreased in the group with polyadenopathies (40.0 +/- 4.0), in the group with AIDS-related complex (28.8 +/- 2.9), and in patients with AIDS (20.4 +/- 0.9). The decrease in BHL was associated with a decrease in blood basophil number.