As a possible more rapid test of lymphocyte reactivity to phytohaemagglutinin (PHA) than the standard estimation of thymidine uptake after 72 hours (T72), uridine uptake after 24 hours (U24) was studied in a total of 96 individuals. No reliable correlation between the two estimations was found in 29 outpatients showing a wide range of reactivity to PHA nor in 30 healthy control subjects nor in 16 pregnant subjects. Conflicting reports have appeared in the literature as to whether lymphocyte reactivity to PHA is normal, depressed or enhanced in pregnancy. Our group of pregnant subjects showed a significant depression of both U24 (P less than 0-01) and T72 (P less than 0-02) results as compared to normal female controls. Cultures of semipurified lymphocytes prepared by gradient centrifugation on a further 21 normal control subjects showed no better correlation between U24 and T72 results than the above unpurified-leucocyte cultures.