Lymphocyte phosphorylase kinase activities were measured in normal controls and in patients with the sex-linked form of liver phosphorylase kinase deficiency. The reaction due to phosphorylase kinase activity in normal lymphocytes (2.7 X 10(6) in the reaction tube) was found to be linear within 20-60 min at 30 degrees C. The reaction was directly proportional to the concentration of lymphocytes within 1.5 X 10(6)-9.0 X 10(6), at 30 degrees C for 60 min. The phosphorylase kinase activity in normal lymphocytes, which were pre-incubated at 50 degrees C or 95 degrees C for 1 min, decreased to 60% at 50 degrees C and 10% at 95 degrees C of that after pre-incubation at 0 degree C for 1 min. The activity of normal controls was 125 +/- 23.5 U/10(10) lymphocytes. Those of the patients with liver phosphorylase kinase deficiency due to the sex-linked form were 43.5 U in case 1, 54.5 U in case 2, and 51.3 U in case 3, respectively and those of the mothers were within the normal range. These results suggest that phosphorylase kinase in lymphocytes might be form intermediate between liver and muscle phosphorylase kinase.