Secretion of IFN-γ by splenocytes from immunized C3HeB and BALB/c mice over 48 h in vitro in the presence of inactivated Sendai virus, as determined by ELISA. No cytokine (data not shown) was detected in supernatant from splenocytes from non-immune mice of either strain in the presence of inactivated virus, or from immunized mice in the absence of virus (F = 27·7, all P < 0·001). Production of IFN-γ by immunized but unchallenged BALB/c mice was less than that from comparable C3HeB mice (* denotes t > 2·5, P < 0·05 between strains in the same treatment group). After challenge in vivo with virus, the production of IFN-γ by cells from immunized C3HeB mice increased (‡ denotes t > 2·0, P < 0·05 versus other values within the same strain), whereas no increase was observed for BALB/c mice. This increase further distinguishes the cytokine response of C3HeB from BALB/c mice (t > 5·5, P < 0·001 for interstrain comparisons between immunized and challenged mice). , C3HeB; □, BALB/c.