We studied the ultrastructural distribution of Concanavalin A sites of urothelial and milk fat globule membranes in situ in EFA-deficient and control rats using the peroxidase-diaminobenzidine procedure in unfixed and glutaraldehyde-fixed materials and incubating temperatures of 4 degrees C and 37 degrees C of the Con A media. In the glutaraldehyde ('prefixed') bladders the lectin marker was homogeneously distributed at the surface of the urothelial membrane with no difference between experimental and control animals. In unfixed bladders ('non-prefixed') of EFA-deficient rats, Con A sites distribution was also homogeneous and similar to that observed in 'prefixed' bladders. In bladders of controls in the 'non-prefixed' condition incubated at both 4 degrees and 37 degrees C, there was a noticeable change in the distribution of Con A sites consisting in a distinct clumping of Con A sites. In 'prefixed' and 'non-prefixed' cream samples of both EFA-deficient and control animals, there was a similar, uniform distribution of Con A sites at the surface of the milk fat globule membrane. The reported changes seemed to suggest that the fluidity of the urothelial membrane but not of milk fat globules was diminished in EFA-deficient rats. Present data do not permit evaluation of the role of cytoskeletal components and/or the variations of the fluidity of the membranes on the distribution of Con A sites at cell surfaces, as herein reported.