An optimized procedure for the preparation of fabric reinforced polyacrylamide gels for native protein blotting is described. The gels, typically 5% T, 3% C, were internally stabilized with the aid of an AcrylAide-pretreated, hydrophilized polyester fabric, preferably with a 60 microns mesh opening. Ultrathin (120-180 microns) gels were prepared with the flap technique and 500 microns gels with the cassette technique; 500 microns gels with immobilized pH gradients were cast using precision molds and a computer controlled mixing device of four burettes. The fabric reinforced gels could be used either wet or after drying and rehydration. Isoelectric focusing was performed in carrier ampholyte pH gradients or hybrid immobilized pH gradients, supplemented with 1-3% w/v carrier ampholytes. Incorporation of 40-60% w/v glycerol into the gels decisively improved their operational properties. The high glycerol gels, which tolerated field strengths of 900-1700 V/cm for extended periods under steady state focusing conditions, were not afflicted by liquid exudation on the gel surface and showed retarded diffusion of the separated proteins on termination of focusing. By unidirectional capillary blotting, with an intermediate dialysis membrane eliminating bidirectional protein transfer, proteins were blotted to 0.1-0.2 micron pore size nitrocellulose membranes in 10-20 min from ultrathin gels and in 30-60 min from 500 microns gels. Based on quantification of residual protein in the gels after blotting, a transfer efficiency of 60-87% was found for the ultrathin and 53-69% for the 500 microns gels. Semidry electrophoretic blotting was carried out in a modified setup with cooled graphite electrodes. In a continuous Tris-glycine buffer system electrophoretic blotting required only 2-5 min with ultrathin gels and 20 min with 500 microns gels. Marker proteins, including horse spleen ferritin (Mr465,000), could be transferred with 91-96% efficiency.