The cleavage of complement may be an important immunopathologic mechanism in the development of gingival inflammation. Utilizing the experimental gingivitis model, cleavage of C3, C4 and B was assessed in gingival fluid following abstention from oral hygiene. 4 male dental students performed stringent oral hygiene measures until the gingival index approached 0, then refrained from any oral hygiene for 21 days. Gingival fluid, sampled with filter paper strips from the mesial surface of all maxillary premolars at 0, 7, 14, and 21 days, was assayed for C3, C4 and B cleavage by multilayer crossed-immunoelectrophoresis. Clinical indices were assessed following gingival fluid sampling. The subjects, who were plaque-free (PI = 0) at the beginning of the study, showed significant plaque accumulation at day 21 (87% of sites with PI greater than or equal to 2). Approximately 90% of the sites were free from clinical inflammation (GI = 0) at the start, but gingivitis increased with time such that 25% of the sites had GI scores of 2 at day 21. Bleeding on probing to the base of the pocket was not observed at day 0, but was observed at 62% of sites by day 21. Statistical analyses showed that all 3 indices significantly increased with time. The %C3 cleavage increased from a mean of 24% at day 0, to 35%, 45% and then 57% at days 7, 14 and 21, respectively, and both days 14 and 21 demonstrated significantly greater C3 conversion than that seen at day 0. The Spearman rank-order correlation coefficient for %C3 conversion versus time was p = 0.52, significant at the p less than 0.0001 level.(ABSTRACT TRUNCATED AT 250 WORDS)