In crown-gall tumor tissue obtained from leaves of Bryophyllum daigremontianum an adenosine 3':5'-cyclic phosphate (3':5'-cyclic-AMP) degrading activity increases up to 2.5 fold until the fifth day after inoculation with Agrobacterium tumefaciens, declining to the value of the control in the solid tumor. Theophylline up to 1 mmol l(-1) given to wounded leaves of Bryophyllum daigremontianum has no effect on the number of tumors. The effect of higher concentrations given over extended periods can be explained otherwise. Therefore it seems likely that the 3':5'-cyclic-AMP phosphodiesterase (EC 3.1.4.17) has no effect on transformation and growth of crown-gall tumors in Bryophyllum daigremontianum.