Freeze-fracture techniques have been used to study gap junction formation between Novikoff hepatoma cells reaggregated for 0-60 min. Quantitative data have been obtained for the proportion of membrane "interfaces" containing formation plaques with and without gap junctions [FP(+)'s and FP(-)'s, respectively], for the areas and numbers of FP(+)'s and FP(-)'s, and for the numbers and densities of 9- to 11-nm particles in FPs. Methods have been devised to correct these data for FP(+)'s carried over from dissociation. Comparisons have also been made with undissociated Novikoff cells. The results show that new FP(-)'s and FP(+)'s first appear between 5 and 15 min of reaggregation, and increase gradually between 15 and 30 min. Interfaces with FP(-)'s develop more rapidly than those with FP(+)'s. Between 30 and 60 min, FP(-)'s begin to disappear while FP(+)'s continue to increase. The results suggest that many FP(-)'s are converted into FP(+)'s, but some FP(-)'s may "abort." The mean areas of FP(-)'s and FP(+)'s, either per plaque or per interface, show no significant changes with reaggregation time, although there is an increased average number of FPs per interface at later times. There is a progressive increase with time in the number of total 9- to 11-nm particles in FPs per plaque and per interface, with densities of 9- to 11-nm particles also generally increasing. However, FP(+)'s, especially in 15-min samples, have a relatively constant density of unaggregated 9- to 11-nm particles, in spite of a large variation in aggregated particle density. A number of related analyses have led us to suggest that the 9- to 11-nm particles in FP(+)'s behave as though they are in a saturated solution, with the aggregated particles being analogous to a precipitate. Particle aggregation in FP(-)'s is considered analogous to precipitation from a supersaturated solution. The relation of the results to previous physiological data is discussed.