In this study we analyzed the activity and the expression of p56^lck protein tyrosine kinase in peripheral blood lymphocytes (PBLs) from systemic lupus erythematosus (SLE) patients and from healthy donors. The p56^lck activity, determined by a non-radioactive Tyrosine Kinase Assay Kit, was significantly higher in active SLE PBLs and discriminated this group of patients from inactive SLE patients (p = 0.002) and healthy donors (p = 0.009). p56^lck level decreased in SLE lymphocytes (especially for inactive SLE lymphocytes, p = 0.005) when compared to healthy donors. These differences were also reflected by the specific activity of p56^lck that was clearly elevated in active SLE lymphocytes when compared to inactive SLE (p = 0.022) or healthy donors lymphocytes (p = 0.006). A positive correlation between the activity of p56^lck and the tyrosine phosphorylation level in active SLE lymphocytes was found.