The effect of various inhibitors on differentiation (shoot morphogenesis) in callus cultures of Brassica, Datura and Nicotiana has been investigated. Hormone medium without any inhibitor (control), resulted in 6% shoot formation. Addition of inhibitors such as actinomycin D, cordycepin, abscisic acid, trigonelline and theophylline greatly enhanced shoot formation. The results suggest that inhibitors play a regulatory role in the control of differentiation.