Administration of allyl isothiocyanate (AITC) and phenyl isothiocyanate (PITC) was found to stimulate the immunological response in Balb/c mice. Treatment with five doses of AITC and PITC (25 microg/dose/animal, i.p.) was found to enhance the total WBC count (13,100 cells/mm3 for AITC) on 9th day and (10,316 cells/mm3 for PITC) on 12th day respectively. Bone marrow cellularity (22.36 x 10(6) cells/femur for AITC, 21.3 x 10(6) cells/femur for PITC) as well as alpha-esterase positive cell number also increased after treatment with AITC (1397 cells/4000 cells) and PITC (1238 cells/4000 cells). Treatment with AITC and PITC along with the antigen sheep red blood cells (SRBC) produced an enhancement in the circulating antibody titer and the number of plaque forming cells (PFC) in the spleen. Maximum number of PFC (1329PFC/10(6) spleen cells) for AITC and PITC (1218PFC/10(6) spleen cells) was obtained on the 5th day. These results indicate the immunomodulatory activities of the two naturally occurring isothiocyanates namely, AITC and PITC.