diff --git a/phasmplatform/common/value.py b/phasmplatform/common/value.py
new file mode 100644
index 0000000..1ba44ea
--- /dev/null
+++ b/phasmplatform/common/value.py
@@ -0,0 +1,26 @@
+from typing import Any, Generic, TypeVar
+
+T = TypeVar('T')
+
+
+class BaseValue(Generic[T]):
+    __slots__ = ('data', )
+
+    data: T
+
+    def __init__(self, data: T) -> None:
+        self.data = data
+
+    def __eq__(self, other: Any) -> bool:
+        return self.__class__ is other.__class__ and self.data == other.data
+
+    def __repr__(self) -> str:
+        return f'{self.__class__.__name__}({repr(self.data)})'
+
+
+class UntypedValue(BaseValue[Any]):
+    pass
+
+
+class BytesValue(BaseValue[bytes]):
+    pass
diff --git a/phasmplatform/worker/__main__.py b/phasmplatform/worker/__main__.py
index 0788256..5d4e366 100644
--- a/phasmplatform/worker/__main__.py
+++ b/phasmplatform/worker/__main__.py
@@ -1,7 +1,10 @@
+from typing import Any
+
 import sys
 
 from phasmplatform.common.config import from_toml
 from phasmplatform.common.router import StdOutRouter
+from phasmplatform.common.value import BaseValue, BytesValue
 
 from .runners.base import BaseRunner
 from .runners.wasmtime import WasmTimeRunner
@@ -27,6 +30,15 @@ def main() -> int:
 
     foo.handle_message(namespace, topic, kind, body)
 
+    inp = BytesValue(b'Hello, world!')
+    print('inp', inp)
+
+    def on_respond(out: BaseValue[Any]) -> None:
+        print('out', out)
+        assert out == inp
+
+    foo.do_call('echo', [inp], on_respond)
+
     return 0
 
 
diff --git a/phasmplatform/worker/runners/wasmtime.py b/phasmplatform/worker/runners/wasmtime.py
index 6e937f8..45de585 100644
--- a/phasmplatform/worker/runners/wasmtime.py
+++ b/phasmplatform/worker/runners/wasmtime.py
@@ -1,9 +1,12 @@
+from typing import Any, Callable, List, Union
+
 import ctypes
 import struct
 
 import wasmtime
 
 from phasmplatform.common.router import BaseRouter
+from phasmplatform.common.value import BaseValue, BytesValue, UntypedValue
 from .base import BaseRunner
 
 
@@ -63,6 +66,21 @@ class WasmTimeRunner(BaseRunner):
 
         return raw[ptr + 4:ptr + 4 + length]
 
+    def convert_value(self, val: BaseValue[Any]) -> Union[int, float]:
+        if isinstance(val, BytesValue):
+            return self.alloc_bytes(val.data)
+
+        raise NotImplementedError(val)
+
+    def do_call(self, method_name: str, args: List[BaseValue[Any]], callback: Callable[[BaseValue[Any]], None]) -> None:
+        method = self.exports[method_name]
+        assert isinstance(method, wasmtime.Func)
+
+        act_args = [self.convert_value(x) for x in args]
+        result = method(self.store, *act_args)
+
+        callback(UntypedValue(result))  # TODO: This returns a bytes pointer, but we can't detect that in advance
+
     def handle_message(self, namespace: bytes, topic: bytes, kind: bytes, body: bytes) -> None:
         namespace_ptr = self.alloc_bytes(namespace)
         topic_ptr = self.alloc_bytes(topic)