To determine the effects of purine antimetabolitcs on the various developmental stages of cells, involved in in vitro carrier and hapten responses, purine antimetabolites were added on day 0, day 1, day 2 or day 3 of the in vitro immune response. Anti-HRBC and anti-TNP responses were assessed on day 2, day 3 and day 4 of culture. Our results indicated that purine antimetabolites were: (1) most effective on early and intermediate precursor cell types, (2) most effective when added early during the in vitro immune response and (3) more effective in inhibiting immunogen-stimulated immune response than nonspecific enhancement of the in vitro immune response.