Frozen and formalin-fixed paraffin-embedded tissue sections were studied concurrently in 17 cases of granulomatous lesions of different etiologies using antibodies recognizing either fixation-sensitive or fixation-resistant antigens. In fixed tissues, the antibodies 4KB5 and L26 for B cells and UCHL1 and MT1 for T cells gave results similar to those obtained in frozen tissues with anti-leu-12/leu-14 for B cells and T-3 for T cells. Paraffin-embedded sections from 35 additional cases of granulomatous lesions were studied retrospectively using the same markers. The combined results from all 52 cases show that granulomas can be divided into two main "families" according to the presence or absence of B cells within the granulomas: one is a B-cell-negative family of lesions to which sarcoidosis and mycobacterial infection belong; the other is a B-cell-positive family of lesions to which toxoplasmosis, granulomatous lesions of unknown significance and tumor-related sarcoid reactions belong.