Cutaneous T- and B-cell responses were studied in three groups of cattle (n=4) after a primary (G1), secondary (G2) and tertiary (G3) experimental infestation with Hypoderma lineatum first instars (L1). Cattle were each infested with 25 L1 deposited onto the skin. Skin biopsies were taken at 0, 6, 12, 48, 96 and 144 h post infestation (hpi). The kinetics of infiltration of CD3(+), CD4(+) and CD8(+) T lymphocytes and B cells at the infestation site was evaluated immunohistochemically. The number and pattern of infiltration of CD3(+) T cells were similar in the different groups of animals, showing a progressive increase until 96 hpi. The number of CD4(+) T helper cells increased significantly at 96, 6 and 48 hpi in G1, G2 and G3, respectively. CD8(+) cytotoxic T-cell infiltration was similar in the different groups, except at 48 hpi when there was a significant difference between G1 and G3. The CD4:CD8 ratio showed predominance of CD4(+) cells throughout the response. The number of B cells increased with the number of previous infestations, with significant differences between G1 and G3 at 12 hpi. All of the cell types concentrated around the superficial and deep dermal vessels. During primary larval infestations there was a predominance of CD4(+) cells, whereas in cattle sensitized by previous infestations, B cells were most abundant in the infiltrate. These findings suggest that humoral immunity may play a significant role in bovine immunity to H. lineatum.