AbstractPhytohemagglutinin (PHA)‐stimulated lymphocyte cultures of a healthy human donor, treated in vitro with 0.5, 1.0, and 1.5 mg LiCl/10 ml of culture (equivalent to 1.1, 1.5, and 2.1 mEq/l, respectively) had an increased incidence of chromosomal anomalies and a significant increase in satellite associations, compared with an unexposed culture. The in vivo effects of Li2CO3on the chromosomes of psychiatric patients on lithium therapy showed increased chromosomal abnormalities, as compared with lithium‐free controls. A significant finding was a marked increase in satellite associations in all lithium‐treated patients. The mitotic index of PHA‐stimulated human lymphocytes exposed to doses of lithium salts (0.5–1.5 mg/10 ml of culture media) was decreased, as measured by [3H]thymidine incorporation i