To assess the significance of humoral immune mechanisms in host reactivity against the P815X2 mastocytoma grown in syngeneic DBA/2 mice, an approach was made to correlate immunomorphology of the thymus and spleen with the functional assays measuring cytotoxic (51Cr-release assay) and tumor cell membrane-bound antibodies (IF test) as well as phenotyping (with monoclonal antibodies Anti-Thy1.2, Anti-Lyt1, Anti-Lyt2, and Anti-I-Ad) the lymphocyte subsets in peripheral blood. The volume fractions (Vv) of the cortex (C), and medulla (M) of the thymus, as well as central (C-PALS) and peripheral periarteriolar lymphoid sheath (P-PALS) in the spleen were determined morphometrically. There was a steady increase of the C/M ratio from the control levels until day 8, not accompanied by any major fluctuations in the percentages of Thy1.2+ or I-Ad+ cells in C and M. In M, two significant peaks were found in Lyt1+/Lyt2+ cell ratio. In the spleen P-PALS, Thy1.2+/I-Ad+ cell ratio was subject to major early elevation, followed by a rapid and permanent decline to levels below the controls. Two high peaks of Lyt1+/Lyt2+ ratio were found in P-PALS, on day 10 and 16. In the C-PALS, a marked decline in Thy1.2+/I-Ad+ ratio was observed throughout the experiment. As determined by Vv, there was a marked early enlargement of P-PALS reaching the peak on day 4, and followed by a steady reduction reaching the control values on day 10. In peripheral blood, there was an initial increase of T-cells, leading to elevated Thy1.2+/I-Ad+ ratio and a subsequent elevation of Lyt1+/Lyt2+ ratio on day 8.(ABSTRACT TRUNCATED AT 250 WORDS)