The in vitro antiproliferative action of pineal indoles on several tumor cell lines including melanoma (B16), sarcoma (S180), macrophage-like cell line (PU5), fibroblasts (3T3), and choriocarcinoma (JAr) was examined by measuring the incorporation of 3H-thymidine by the tumor cells, and, in the case of melanoma cells, by also measuring the incorporation of 3H-leucine and 3H-uridine. Uptake of crystal violet was used to assess the viability of the tumor cells. The order of inhibitory potency of the indoles was found to be methoxytryptamine > melatonin, methoxytryptophol, hydroxytryptophol, and methoxyindoleacetic acid > serotonin and hydroxyindoleacetic acid. The possibility of an adverse effect of the indoles on the viability of normal cells was also investigated by employing a primary culture of rat hepatocytes. The release of glutamate-oxaloacetate transaminase by hepatocytes was not affected by the indoles, although the release of glutamate-pyruvate transaminase was increased to a small extent and the uptake of crystal violet was slightly inhibited.