Monotonic convergence is established for a general class of multiplicative algorithms introduced by Silvey, Titterington and Torsney [Comm. Statist. Theory Methods 14 (1978) 1379.1389] for computing optimal designs. A conjecture of Titterington [Appl. Stat. 27 (1978) 227.234] is confirmed as a consequence. Optimal designs for logistic regression are used as an illustration.