Parameters of the cell cycle of lymphoid cells and the rate of production of small lymphocytes and mature plasma cells were estimated in human spleens. 12 normal and 1 enlarged spleen with lymphatic hyperplasia were labelled either by a pulse or continuously with 3H-TdR-thymidine during normothermic perfusion of the isolated spleens. Biopsies of splenic tissue were taken at different intervals and evaluated autoradiographically. The mean initial labelling index of immunoblasts was 73% and for plasmoblasts 52%. The labelling index for small lymphocytes and mature plasma cells increased initially from 0% to 1%, and up to 25% respectively after 14 h. The duration of the S-phase, as determined by double labelling, was 8.26 h for lymphoid blasts. Data from the percentage labelled mitosis curve resulted in a minimal tG2 of about 1 h and tG2 + tM of about 4 h. The estimated generation time was about 11.5 h. The results found on the hyperplastic spleen were comparable to those on the normal spleen.