We prospectively studied 42 adult patients with acute dermis and soft-tissue infections (27 with erysipelas and 15 with acute cellulitis) involving the lower limb in all except one case. Streptococcus organisms (groups A, C, D, and G) were researched in skin biopsy specimens by a direct immunofluorescent (DIF) technique using commercially available antibodies. Our results showed that DIF gives a sensitivity of 0.70 for the in situ detection of streptococci in cases of erysipelas and cellulitis. With the obvious contribution of this DIF technique, streptococcal pathogens could be detected in situ and grouped in 19 of 27 cases of erysipelas (group A, 13; group B, 1; group C, 1; and group G, 4) and in ten of 15 cases of cellulitis (group A, 9; group B, 1). Combined data, including conventional cultures, DIF studies, and serologic findings, established that Streptococcus organisms, especially Streptococcus pyogenes (A), were, in nearly all cases, responsible for both erysipelas (26/27 cases) and acute cellulitis (11/15 cases) involving the lower limb in adults.