The presence of cytoplasmic material inside thin Millipore filters between interacting mouse metanephric mesenchyme and spinal cord was investigated using different fixation methods. The transmission of induction was studied from sections of Zenker-fixed paraffin-embedded explants. Formation of kidney tubules was taken as evidence for induction. Filters with 0.8 and 0.22 mum pores permitted induction, whereas only 6 out of 31 filters with 0.1 mum pores did so. Glutaraldehyde-fixed Epon-embedded explants were used for study of cytoplasmic penetration into filters. In thick sections, filters with large pores were seen to contain cytoplasmic material at all levels. Filters with 0.1 mum pores usually showed only shallow ingrowth, but those which had permitted passage of induction contained material at least half way from the spinal cord and shallow ingrowth from the mesenchyme. With 0.8 mum filters the ingrowths from both sides met first after 18 h of transfilter cultivation. This has previously been shown to be the minimum time needed for induction to take place in this system. In electron microscopy cytoplasmic processes were seen deep inside the 0.8 and 0.22 mum filters regularly permitting induction. In small pores such material was only preserved by certain glutaraldehyde fixatives. Diffusion studies did not reveal major differences between induction-permitting 0.22 mum filters and induction-preventing 0.1 mum filters. Thus in the kidney tubule induction system this and our previous work speak in favor of a mechanism based on close apposition of cells rather than on long-range diffusion of inductive substances or on matrix interaction.