We aimed to evaluate the role of the CD19 complex in the pathogenesis of transient hypogammaglobulinemia of infancy (THI) and to better characterize the subsets of memory B cells. The study population consisted of 22 male and 14 female patients with a mean age at presentation of 20 ± 9.9 months. The CD19 complex and B cell subsets were evaluated by flow cytometry. While the CD19 median fluorescence index (MFI) in patients with THI was significantly lower than controls (122.9 ± 66.7 in patients; 184.2 ± 39 in controls, p < 0.01), expression of CD21 and CD81 was increased (94.4 ± 3, 96.8 ± 2.5 % in patients; 91 ± 3.9; 94.7 ± 3.5 % in controls, p < 0.01 vs. p < 0.05, respectively). The expressions of switched memory B cells and IgM memory B cells were found to be reduced in THI. Considering that the CD19 complex regulates the events following antigen stimulation, the change in CD19 complex detected in THI may be related to insufficiency of antibody production. © 2012 Springer-Verlag.