A new form of chamber for studying chemotaxis, similar in principle to the Zigmond chamber, allows the behaviour of the cells in a linear concentration gradient to be observed directly. The chamber was developed mainly for studying chemotaxis in fibroblasts using interferometric microscopy and the main design criteria were that it should have better optical characteristics, a higher dimensional precision and better long-term stability than the Zigmond chamber. It is made entirely from glass by grinding a blind circular well centrally in the counting platform of a Helber bacteria counting chamber. This procedure leaves an annular 'bridge', approximately 1 mm wide, between the new inner circular well and the original outer annular well. This bridge fulfils the same function as the linear bridge of the Zigmond chamber but the precise construction of the counting chamber ensures that a gap of 20 microns between bridge and coverslip can be accurately and repeatedly achieved when the chamber is assembled. It is envisaged that the improved optical clarity, dimensional accuracy and long-term stability of the new chamber will be advantageous in other applications, particularly in studies requiring critical microscopy or a precise knowledge of the gradient and in studies of cells, such as fibroblasts, that move much more slowly than neutrophils.