A series of 1-thia analogues of the pyrrolizine bis(carbamate) 9 (NSC-278214), namely 5-aryl-2,3-dihydropyrrolo-[2,1-b]thiazole-6,7-dimethanol 6,7-bis(isopropylcarbamates) (7a-d), were prepared by multistep syntheses from the proline analogue thiazolidine-2-carboxylic acid. The compounds were tested for growth inhibitory activity with the HL-60 human promyelocytic leukemia cell line. Three of the compounds had antileukemic activity equal to that of 9, while a 4-chlorophenyl analogue was approximately 75% more potent. A simple aromatic derivative, 1,2-benzenedimethanol 1,2-bis(isopropylcarbamate) (8), had no activity in this system. Antitumor activity was also tested in a colony formation assay with HT-29 human colon carcinoma cells. Compounds 7a-d reduced relative cell survival by over 3 logs at a concentration of 300 .mu.M (2-h exposure), while a comparable inhibition was observed with 150 .mu.M 9. Hence compounds 7a-d retain significant antineoplastic activity.