B cell stimulation develops upon vaccination, thus causing occurrence of activated B cells (plasmoblasts) in bloodstream. Similar cells are also observed in some viral infections. The contents of plasmablasts may be a marker of successful vaccination, or a diagnostic feature of ongoing infection. The plasmablasts are normally represented by a small cell subpopulation which is not easy to detect. A study was performed with 15 healthy volunteers who were subjected to a single immunization with a recombinant vaccine against hepatitis B virus. To identify the plasmablasts, we have used labeled antibodies prepared in our laboratory. These reagents were previously validated for counting the plasmablasts. Different gating strategies for plasmablast gating have been compared. Upon staining of lymphocytes from immunized volunteers, we observed a distinct cluster of plasmablasts with CD27++CD38++ phenotype using the following antibody set: CD19-PE, CD3/CD14/CD16-FITC, CD27-PC5.5 and CD38-PC7. Inclusion of a CD20-FITC antibody into the panel caused an increase of CD27++CD38++ plasmablast ratio among CD19+ lymphocytes to > 60%. Upon substitution of CD38 antibody by anti-CD71, a distinct plasmablast cluster was again revealed, which contained ca. 5 per cent B cells. Two strategies for the plasmablast gating using the CD27/ CD38 and CD27/CD71 combinations were compared in dynamics with lymphocyte samples from a single vaccinated volunteer. When applying the CD27/CD38 combination, a sharp and pronounced plasmablast peak was registered on day 7 post-vaccination. With CD27/CD71 combination, the peak was extended between day 7 and day 14 following immunization. Hence, time kinetics of the CD27+CD71+ population proved to be different from occurrence of classic plasmablasts with CD27++CD38++ phenotype. This finding suggests that the CD27++CD71+ population contains both plasmablasts and other types of activated B cells. A minor HBV surface antigen was prepared and labeled with phycoerythrin (HBsAg-PE), thus allowing to quantify the antigen-specific plasmablasts. The results of HBsAg-PE-based detection of antigen-specific cells were in compliance with the data obtained by ELISpot technique. At the present time, we use the original plasmablast gating technique for detection of activated B cells in SARS-CoV-2 infection. At the next step, this technique will be applied to sorting of antigen-specific B cells, thus permitting sequencing of Ig genes and design of novel human antibodies against viral antigens. © 2020, SPb RAACI.