Cell kinetic variables in human tumours were investigated before and during therapy. Tumour cell material was collected by sequential thin needle aspiration biopsy. Small variations in DNA replication could be detected by 3H-thymidine autoradiography. In tumours with a high rate of replication, therapy induced changes could also be detected with flow cytometry and static cytophotometry. Endocrine treatment with glucocorticoids, radiation therapy, and chemotherapy were associated with reduced rates of DNA replication in malignant lymphomas. The conformity between labelling index and S-phase estimates from DNA histograms was poorer in biopsy specimens collected during therapy than in pretreatment specimens.