Human leukocyte concentrates recovered from leukocyte reduction system (LRS) chambers of the Trima Accel automated blood collection device are a by-product of platelet apheresis that is often used in research. The Trima Accel software was obligatorily updated in 2023 from version 6 to 7. Here, we investigated software-dependent differences in the cellular composition of LRS concentrates when performing apheresis with either plasma or platelet additive solution. When using plasma as suspension medium, the software update to Trima 7 led to higher fraction of B cells as revealed by flow cytometry. Compared to platelets-in-plasma collection, the total recovered volume and leukocyte density was significantly reduced when running Trima 7 software with platelet additive solution including a plasma rinseback. Moreover, the proportion of lymphocytes and monocytes in these products was lower, whereas the proportion of neutrophils and eosinophils was higher. Researchers working with leukocytes isolated from LRS chambers should be aware that performing apheresis with platelet additive solution alters the composition of their starting material, which could affect the interpretation of some experiments.