Prospective biochemical, virological and selected immunological follow up has been done for up to 32 months on 15 previously untreated haemophilic boys following treatment with an intermediate purity dry heated factor VIII concentrate (BPL 8Y). Tests for liver function and antibodies to blood-borne viruses have been assessed monthly for the first year after starting treatment and thereafter every 2 months. All patients were immunized against hepatitis B and have not developed hepatitis B core antibodies and no boy has shown any rise in alanine transaminase level nor has anyone developed antibodies to hepatitis C (HCV). All patients have remained anti-HIV seronegative. T lymphocyte subsets have been measured approximately every 4 months and in no patient has there been a significant rise in CD8+ cells; one patient showed a significant decrease in CD4+ cells but these and all CD4+ values for the other boys remained within normal age related limits. Changes in CD4+ levels in this one boy were not related to the total amount of treatment received. This group of patients who appear not to have contracted HIV, hepatitis B or non A non B hepatitis following treatment with this intermediate purity factor VIII concentrate have also not shown any consistent changes in CD4+ or CD8+ cells, which have been recorded previously in frequently treated haemophiliacs.