In a longitudinal analysis of immunological changes in normal human pregnancy, twenty-two pregnant women were studied, with blood samples taken before pregnancy, during its course, at delivery, and 3–5 months after delivery. The blood samples were frozen successively, using a cryobiological freezing system, and were stored until the whole longitudinal series was obtained. The collected material for a longitudinal series was then thawed and tested in one seance. Lymphocyte transformation tests were performed with stimulation with the mitogens phytohae-magglutinin (PHA), pokeweed mitogen (PWM), concanavalin A (Con A) and the specific antigens tuberculin purified protein derivative (PPD), Candida albicans (CA), Staphylococcus aureus (SA) and streptokinase/streptodornase (SK/SD). No changes were found in PHA or in PWM responses, whereas there were significant changes in the Con A response, with lowest values in the middle of pregnancy. PPD, CA, SA and SK/SD responses all reached lowest values towards the end of pregnancy, with increases subsequently, after delivery. These findings seem to show that T suppressor function is increased and B-lymphocyte function decreases during the course of pregnancy.