The results of two recent studies of our group have been reported. They regard two immunological parameters of psoriatic arthritis: the proportions of T gamma lymphocytes in peripheral blood and the beta 2 microglobulin in the serum. The data obtained in psoriatic arthritis patients have been compared to those found in normal controls and in rheumatoid arthritis patients. T gamma mean values in psoriatic arthritis were significantly lower than those present in healthy subjects and in rheumatoid patients. These last patients showed beta 2 microglobulin mean values significantly higher than those observed in normal controls and in psoriatic arthritis patients. Conversely, the mean of beta 2 microglobulin levels in psoriatic arthritis has been found to be similar to that observed in normal controls, but a superimposition in the range of individual values of these two groups with the concentrations determined in rheumatoid subjects has been found. These results seem to be of interest in relation to the immunopathogenetic mechanism of psoriatic arthritis, but are of little help in the clinical differentiation of the two rheumatological affections considered.