A single intratracheal injection of bleomycin in rats induced, 4 days later, a considerable accumulation of hyaluronan (hyaluronate, hyaluronic acid) in the lung tissue. This connective tissue reaction was quantified biochemically by analysing hyaluronan (HA) in bronchoalveolar lavage fluid (BAL) and lung tissue extracts. The molecular weight of the HA recovered during lavage was 0.2-0.3 X 10(6) daltons. The HA accumulation was related to an increase in lung water content and associated with an increased influx of eosinophils, neutrophils and lymphocytes into BAL fluid. High-dose corticosteroid treatment (prednisolone 15 mg.kg-1 bw per day) to bleomycin injured rats had no effect on the lung tissue content of HA, the recovery of HA during BAL or the molecular weight of HA accumulated in the alveolar space. Furthermore, steroids did not influence the increased lung water content, or the appearance of inflammatory cells in lavage fluid. These findings indicate that the early connective tissue response to the bleomycin lung injury is mediated by a mechanism which is unaffected by systemic high-dose corticosteroids.