The polarity of fucoid eggs is fixed either when tip growth starts or a bit earlier. A steady flow of calcium ions into the incipient tip is thought to establish a high calcium zone that is needed for its localization and formation. To test this hypothesis, we have injected seven different 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-type calcium buffers into Pelvetia eggs many hours before tip growth normally starts. Critical final cell concentrations of each buffer prove to block outgrowth (as well as cell division) for up to 2 weeks. This critical inhibitory concentration is lowest for two buffers with dissociation constants or Kd values of 4-5 x 10(-6) M and increases steadily as the buffers' Kd values shift either below or above this optimal value to ones as low as 4 x 10(-7) M or as high as 9.4 x 10(-5) M. To analyze these results, we have derived an equation (based on the concept of facilitated diffusion) for the effects of diffusable calcium buffers on steady-state calcium gradients. The data fit this equation quite well if it is assumed that cytosolic free calcium at the incipient tip is normally kept at about 7 microM and, thus, far above the general cytosolic level.