It has been demonstrated that HIV infection may affect the levels of thymidine kinase (TK) and deoxycytidine kinase (dCK) in peripheral blood mononuclear cells from HIV infected adults. The aim of this study was to examine the effect of HIV infection and/or antiretroviral therapy on the activity of the above enzymes in HIV-infected children. The results showed that an inter-individual variability in TK and dCK activities does exist in both HIV infected and uninfected children. TK and dCK levels in PBMC from HIV infected and non infected children did not significantly differ. Furthermore, the therapeutic regimen, including zidovudine, does not seem to affect TK activity.