In 19 children with hepatic fibrosis as the result of continued schistosomiasis mansoni and 20 children without hepatic fibrosis, the following studies were carried out: HLA antigen typing for 30 antigens, immune response of T lymphocytes to schistosome antigen by measuring DNA synthesis evidenced by 3H-thymidine uptake, and measurement of total OKT3+, OKT4+, and OKT8+ cells using monoclonal antibodies. Patients with hepatic fibrosis were mostly high responders in contrast with those without fibrosis. High immune response and susceptibility to post-schistosomal hepatic fibrosis were associated with a high frequency of A2 and B12 antigens and a lack of DR2 antigens, while low response was associated with the presence of the DR2 antigen. The T4+:T8+ ratio showed increased suppressor proportions in patients with low immune response and/or with no hepatic fibrosis. We suggest an immunogenetic susceptibility for post-schistosomal hepatic fibrosis, probably controlled by HLA-linked genes via the suppressor T cells.