The state of cell-mediated immunity was measured by the morphological method of lymphocyte transformation with and without PHA, in a group of 56 patients with bacterial infections and in a group of healthy controls. The patients were divided into three subgroups according to the aetiology: 1. Patients with Gram-positive infections, 2. Patients with Gram-negative infections, and 3. mixed infections. The transformation values with PHA were significantly (t-test) higher in the Gram-positive subgroups than in the control group. Transformation in cultures stimulated by PHA was much higher in the subgroup of patients with Gram-positive aetiology; the difference between the values in the Gram-negative and in the Gram-positive subgroups was also significant. A decrease of transformation value below 50% was observed in 6 out of 18 patients with Gram-negative aetiology, whereas in the Gram-positive group it was noted in only 2 out of 21 patients.