Tube growth rate, wall biosynthesis and wall polysaccharide composition were studied in pollen of Malus domestica Borkh. germinating in the presence of monensin, a monovalent ionophore which is known to cause swelling of Golgi apparatus cisternae in plant and animal cells. 125 nM monensin decreased tube growth rate, inhibiting tube elongation by 50% within 2 h germination. Wall biosynthesis was reduced in the treated tubes: label incorporation decreased to 61% (acid-soluble fraction) and 54% (alkali-soluble fraction) with respect to controls. Protein content of the wall ionic fraction was also decreased in the presence of the ionophore. Moreover, the sugar composition of acid or alkali-soluble wall fractions was shown to be partially affected by monensin; this could result from a decrease in number and reactivity of the large vesicles derived from dictyosomes, which was previously observed in treated apple pollen tubes.