AbstractThe effects of 10 fixatives and formalin fixation time were assessed on the immunoreactivity of KP1 (CD68 antigens) in colorectal tissues in paraffin embedded sections. A panel of fixatives including B5, periodate-lysine-paraformaldehydedichromate, periodate-lysine-paraformaldehyde, 4% paraformaldehyde (PF), zinc formalin, formol dichromate, 10% neutral buffered formalin (NBF), Bouin fluid, Carnoy fluid, and acetone were used to determine an optimal fixative. Another set of colorectal tissues were fixed in 10% NBF progressively at intervals of 1,2,4, and 20 hr; 2,7, and 14 days; and 4 wk to investigate the formalin-induced detrimental effect on CD68 antigens of macrophages.The best result was found after fixation in B5. Short fixation in NBF for 4 hr with trypsinization also gave satisfactory results. There was a distinct decrease in staining intensity after 20 hr exposure to NBF, and the reactivities were totally abolished after 2 days. Trypsinization did enhance the staining intensity significantly with no discrimination of fixatives. (The J Histotechnol17:329, 1994)