AbstractThe effect of low-dose mechlorethamine (5μg/kg) on secondary humoral response to sheep red blood cells (SRBC), depending on time of exposure to the drug in relation to priming and challenge was studied in Balb/c mice. It was found that mechlorethamine in a dose of 5μg/kg stimulated primary humoral response to SRBC resulting in the increased number of the plaque forming cells (PFC) and hemagglutinin titre (19S + 7S). However, this effect waned 10 days after immunization. On the other hand, the same mechlorethamine dose potentiated secondary humoral response to SRBC and increased the number of PFC and anti-SRBC hemagglutinin titres (notably 7S), which was due to the challenging antigenic stimulus. In each immunization, mechlorethamine administration prolonged the potentiating effect of the drug on anti-SRBC hemagglutinin titre. When mechlorethamine was administered to the mice only after priming, the number of PFC increased, but anti-SRBC hemagglutinin titre (7S) remained unchanged. This was likely due to the fact that mechlorethamine administered after priming increases the number of long-lived lymphocytes B, which in turn affect secondary humoral response.