In this paper, it is assumed that tissue fixation is a process in which the proteins become less soluble and catabolic reactions stop. With this definition in mind, 2.5 and 5% glutaraldehhde and 4% formaldehyde in 0.1 M potassium phosphate buffer, pH 7.4, were compared with a new fixative, bicarbonate-formaldehyde. The following results were obtained. (I) With 2.5 and 5% glutaraldehyde, the solubility of tissue proteins were not decreased unifromly, and tissue glycogen was poorly preserved. (2) 4% formaldehyde in potassium phosphate buffer gave relatively good results. (3) Bicarbonate-formaldehyde decreased the solubility of tissue proteins reliably and preserved tissue glycogen perfectly. Histologically, it yielded excellent results. Since glutaraldehyde alters the properties of proteins substantially (Hopwood, 1972; Habeeb & Hiramoto, 1968), and since the natural appearance of tissues depends on native tissue proteins, formaldehyde-containing fixatives, in particular bicarbonate-formaldehyde, are preferable to glutaraldehyde-containing fixatives for all tissue preparative techniques. However, it is important that the fixation time in formaldehyde is kept short.