Centre formation in Polysphondylium violaceum is delayed for 2 h on buffered agar containing 10(-3) M c-AMP, and for up to 22 h on unbuffered agar with the same c-AMP concentration. With ambient c-AMP concentrations as low as 10(-6) M, P. pallidum forms numerous, small, atypical aggregates which do not fruit. This effect is independent of whether the agar is buffered. P. violaceum amoebae are weakly attracted to the tip of a microelectrole containing 10(-3) or 10(-4) M c-AMP, but the electrode cannot compete when natural centres form nearby. P. pallidum amoebae are not attracted. Aggregates of P. violaceum and of P. pallidum are strongly attracted to a microelectrode releasing c-AMP. The observation of Shaffer that Polysphondylium grex switch over from secreting an acrasin that attracts homologous amoebae to one that attracts the larger Dictyostelium species suggests that the second acrasin might by c-AMP. The above results strengthen this conjecture. As c-AMP inhibits centre formation, the secretion of c-AMP by older aggregates may explain an inhibition of centre formation in the 'overlay' experiments of Shaffer.